<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019f3d79-9e9a-7e04-b7d7-b7e86c7f5265,g:bb857668-f3c3-4a7c-9add-2120192d81d1-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/COVER" xlink:type="simple" xlink:href="toi-20260630.xsd#COVER"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/COVER" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_019f3d79-a01d-7e45-a738-da7e945252cb" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_DocumentInformationTable_019f3d79-a01d-7e45-a738-da7e945252cb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019f3d79-a01d-78c0-9692-61086fc00bf4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_019f3d79-a01d-7e45-a738-da7e945252cb" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019f3d79-a01d-78c0-9692-61086fc00bf4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f3d79-a01d-78c0-9692-61086fc00bf4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f3d79-a01d-78c0-9692-61086fc00bf4" xlink:to="loc_us-gaap_ClassOfStockDomain_019f3d79-a01d-78c0-9692-61086fc00bf4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f3d79-a01d-792c-852f-7e7b0efe5e2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f3d79-a01d-78c0-9692-61086fc00bf4" xlink:to="loc_us-gaap_ClassOfStockDomain_019f3d79-a01d-792c-852f-7e7b0efe5e2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019f3d79-a01d-7ef7-8f82-f335c160db46" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f3d79-a01d-792c-852f-7e7b0efe5e2a" xlink:to="loc_us-gaap_CommonStockMember_019f3d79-a01d-7ef7-8f82-f335c160db46" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_019f3d79-a01d-799d-8bcf-5f7d7ee1caef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f3d79-a01d-792c-852f-7e7b0efe5e2a" xlink:to="loc_us-gaap_WarrantMember_019f3d79-a01d-799d-8bcf-5f7d7ee1caef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_019f3d79-a01d-7ad5-8d15-253d97d0f140" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_DocumentType_019f3d79-a01d-7ad5-8d15-253d97d0f140" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_019f3d79-a01d-7527-aa5a-5d03a1a7f854" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_DocumentQuarterlyReport_019f3d79-a01d-7527-aa5a-5d03a1a7f854" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_019f3d79-a01d-7c89-8193-95ebf2c1da2e" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_DocumentPeriodEndDate_019f3d79-a01d-7c89-8193-95ebf2c1da2e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_019f3d79-a01d-7dfa-8d30-50391595be71" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_DocumentTransitionReport_019f3d79-a01d-7dfa-8d30-50391595be71" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_019f3d79-a01d-7d42-b4eb-efe787fc03a0" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityFileNumber_019f3d79-a01d-7d42-b4eb-efe787fc03a0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_019f3d79-a01d-702a-ac59-3a393ed59988" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityRegistrantName_019f3d79-a01d-702a-ac59-3a393ed59988" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_019f3d79-a01d-7615-ba39-47153373dfa5" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityIncorporationStateCountryCode_019f3d79-a01d-7615-ba39-47153373dfa5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_019f3d79-a01d-74fd-a334-4cdccbe48cf3" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityTaxIdentificationNumber_019f3d79-a01d-74fd-a334-4cdccbe48cf3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_019f3d79-a01d-78c9-bc9c-14cc267fdf50" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityAddressAddressLine1_019f3d79-a01d-78c9-bc9c-14cc267fdf50" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_019f3d79-a01d-7357-a522-2aa15e57f148" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityAddressCityOrTown_019f3d79-a01d-7357-a522-2aa15e57f148" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_019f3d79-a01d-74c7-8491-a6b1136c3cbc" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityAddressStateOrProvince_019f3d79-a01d-74c7-8491-a6b1136c3cbc" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_019f3d79-a01d-7885-acf4-f8a1d8c5c887" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityAddressPostalZipCode_019f3d79-a01d-7885-acf4-f8a1d8c5c887" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_019f3d79-a01d-77e7-bf9c-b5c6cf21efcf" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_CityAreaCode_019f3d79-a01d-77e7-bf9c-b5c6cf21efcf" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_019f3d79-a01d-7d28-a224-6a77330c691a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_LocalPhoneNumber_019f3d79-a01d-7d28-a224-6a77330c691a" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInformationFormerLegalOrRegisteredName_019fd258-53c5-700f-8951-46343855e464" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityInformationFormerLegalOrRegisteredName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityInformationFormerLegalOrRegisteredName_019fd258-53c5-700f-8951-46343855e464" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_019f3d79-a01d-750b-a96b-ffeabb27fa6d" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_Security12bTitle_019f3d79-a01d-750b-a96b-ffeabb27fa6d" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_019f3d79-a01d-76f7-899c-bfb382a968a8" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_TradingSymbol_019f3d79-a01d-76f7-899c-bfb382a968a8" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_019f3d79-a01d-739a-abee-ac00ac644bf2" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_SecurityExchangeName_019f3d79-a01d-739a-abee-ac00ac644bf2" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_019f3d79-a01d-703f-868e-e996eaa4bb05" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityCurrentReportingStatus_019f3d79-a01d-703f-868e-e996eaa4bb05" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_019f3d79-a01d-7c95-bee4-990e7a071945" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityInteractiveDataCurrent_019f3d79-a01d-7c95-bee4-990e7a071945" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_019f3d79-a01d-7189-b09a-de587286d1c8" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityFilerCategory_019f3d79-a01d-7189-b09a-de587286d1c8" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_019f3d79-a01d-7791-b9ec-e9b93337f02b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntitySmallBusiness_019f3d79-a01d-7791-b9ec-e9b93337f02b" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_019f3d79-a01d-72e2-8268-78dea9e14d63" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityEmergingGrowthCompany_019f3d79-a01d-72e2-8268-78dea9e14d63" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_019f3d79-a01d-7953-b22a-447748b93a32" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityShellCompany_019f3d79-a01d-7953-b22a-447748b93a32" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_019f3d79-a01d-7a41-a98a-ac471f7c9f45" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_019f3d79-a01d-7a41-a98a-ac471f7c9f45" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_019f3d79-a01d-7da2-a7bc-fc8fa0fdca8c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_EntityCentralIndexKey_019f3d79-a01d-7da2-a7bc-fc8fa0fdca8c" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_019f3d79-a01d-7805-b0e1-6f4cebb0a60b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_CurrentFiscalYearEndDate_019f3d79-a01d-7805-b0e1-6f4cebb0a60b" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_019f3d79-a01d-7dc8-8c34-7fce677a9c40" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_DocumentFiscalYearFocus_019f3d79-a01d-7dc8-8c34-7fce677a9c40" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_019f3d79-a01d-7fb6-9aba-6ca1df6d490b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_DocumentFiscalPeriodFocus_019f3d79-a01d-7fb6-9aba-6ca1df6d490b" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_019f3d79-a01d-7818-8020-7d680cbdc3ad" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_019f3d79-a01d-7b93-80c5-a341e80017e8" xlink:to="loc_dei_AmendmentFlag_019f3d79-a01d-7818-8020-7d680cbdc3ad" xlink:type="arc" order="29"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="simple" xlink:href="toi-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019f3d79-a01d-757b-9d22-740ebf09e510" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_StatementTable_019f3d79-a01d-757b-9d22-740ebf09e510" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019f3d79-a01d-70a2-9158-2f26958682dd" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019f3d79-a01d-757b-9d22-740ebf09e510" xlink:to="loc_srt_ProductOrServiceAxis_019f3d79-a01d-70a2-9158-2f26958682dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f3d79-a01d-70a2-9158-2f26958682dd_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019f3d79-a01d-70a2-9158-2f26958682dd" xlink:to="loc_srt_ProductsAndServicesDomain_019f3d79-a01d-70a2-9158-2f26958682dd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f3d79-a01d-75dd-aa48-db6e7d67462a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019f3d79-a01d-70a2-9158-2f26958682dd" xlink:to="loc_srt_ProductsAndServicesDomain_019f3d79-a01d-75dd-aa48-db6e7d67462a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HealthCarePatientServiceMember_019f3d79-a01d-7b3d-a638-b25b38bcdc90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HealthCarePatientServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a01d-75dd-aa48-db6e7d67462a" xlink:to="loc_us-gaap_HealthCarePatientServiceMember_019f3d79-a01d-7b3d-a638-b25b38bcdc90" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_HealthCareSpecialtyPharmacyMember_019f3d79-a01d-7cf9-aea0-4df02e2365da" xlink:href="toi-20260630.xsd#toi_HealthCareSpecialtyPharmacyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a01d-75dd-aa48-db6e7d67462a" xlink:to="loc_toi_HealthCareSpecialtyPharmacyMember_019f3d79-a01d-7cf9-aea0-4df02e2365da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClinicalResearchTrialsAndOtherRevenueMember_019f3d79-a01d-7113-87b8-186c6da043b1" xlink:href="toi-20260630.xsd#toi_ClinicalResearchTrialsAndOtherRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a01d-75dd-aa48-db6e7d67462a" xlink:to="loc_toi_ClinicalResearchTrialsAndOtherRevenueMember_019f3d79-a01d-7113-87b8-186c6da043b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesAbstract_019f3d79-a01d-71fc-adf5-419515e21c90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_RevenuesAbstract_019f3d79-a01d-71fc-adf5-419515e21c90" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019f3d79-a01d-7906-a58e-aca6642947dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_019f3d79-a01d-71fc-adf5-419515e21c90" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019f3d79-a01d-7906-a58e-aca6642947dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_019f3d79-a01d-7129-9585-7fdcca3f4c32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_OperatingExpensesAbstract_019f3d79-a01d-7129-9585-7fdcca3f4c32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_019f3d79-a01d-7056-bd48-5dd1ee5419de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019f3d79-a01d-7129-9585-7fdcca3f4c32" xlink:to="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_019f3d79-a01d-7056-bd48-5dd1ee5419de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019f3d79-a01d-7d54-a520-5484370358a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019f3d79-a01d-7129-9585-7fdcca3f4c32" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019f3d79-a01d-7d54-a520-5484370358a3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019f3d79-a01d-779a-9cd7-5e609d19d728" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019f3d79-a01d-7129-9585-7fdcca3f4c32" xlink:to="loc_us-gaap_DepreciationAndAmortization_019f3d79-a01d-779a-9cd7-5e609d19d728" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingCostsAndExpenses_019f3d79-a01d-7643-bb15-c3bebb6b4fa2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingCostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019f3d79-a01d-7129-9585-7fdcca3f4c32" xlink:to="loc_us-gaap_OperatingCostsAndExpenses_019f3d79-a01d-7643-bb15-c3bebb6b4fa2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019f3d79-a01d-73b5-b888-0dea9c0619e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_OperatingIncomeLoss_019f3d79-a01d-73b5-b888-0dea9c0619e2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019f3d79-a01d-75f3-b966-56c1405f75ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonoperatingIncomeExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019f3d79-a01d-75f3-b966-56c1405f75ba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_019f3d79-a01d-768a-90f3-9b8367038bc4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019f3d79-a01d-75f3-b966-56c1405f75ba" xlink:to="loc_us-gaap_InterestExpenseNonoperating_019f3d79-a01d-768a-90f3-9b8367038bc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAdjustmentOfWarrants_019f3d79-a01d-79b1-bc5a-f2c944de596d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAdjustmentOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019f3d79-a01d-75f3-b966-56c1405f75ba" xlink:to="loc_us-gaap_FairValueAdjustmentOfWarrants_019f3d79-a01d-79b1-bc5a-f2c944de596d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeGainLossOnDerivativeNet_019f3d79-a01d-7067-b487-b1e6ad74b101" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeGainLossOnDerivativeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019f3d79-a01d-75f3-b966-56c1405f75ba" xlink:to="loc_us-gaap_DerivativeGainLossOnDerivativeNet_019f3d79-a01d-7067-b487-b1e6ad74b101" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019f3d79-a01d-708a-ba0e-5a326246d7dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019f3d79-a01d-75f3-b966-56c1405f75ba" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019f3d79-a01d-708a-ba0e-5a326246d7dc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_019f3d79-a01d-717f-8eb9-553ab658bd8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019f3d79-a01d-75f3-b966-56c1405f75ba" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_019f3d79-a01d-717f-8eb9-553ab658bd8b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019f3d79-a01d-7d19-8f66-675f881b1c03" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019f3d79-a01d-7d19-8f66-675f881b1c03" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019f3d79-a01d-7841-b26e-8c2c6327b0b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019f3d79-a01d-7841-b26e-8c2c6327b0b4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019f3d79-a01d-73aa-9db4-04de62b9d33b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_NetIncomeLoss_019f3d79-a01d-73aa-9db4-04de62b9d33b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_019f3d79-a01d-7b3b-bf2d-e32252ad9f04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_019f3d79-a01d-7b3b-bf2d-e32252ad9f04" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_019f3d79-a01d-71e9-84d4-eeda27c824e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_019f3d79-a01d-71e9-84d4-eeda27c824e0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareAbstract_019f3d79-a01d-7699-9d0f-3cd5e8c58a44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_EarningsPerShareAbstract_019f3d79-a01d-7699-9d0f-3cd5e8c58a44" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_019f3d79-a01d-7910-873d-812a8b3e198b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_019f3d79-a01d-7699-9d0f-3cd5e8c58a44" xlink:to="loc_us-gaap_EarningsPerShareBasic_019f3d79-a01d-7910-873d-812a8b3e198b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_019f3d79-a01d-7d41-aa09-5a180cb069bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_019f3d79-a01d-7699-9d0f-3cd5e8c58a44" xlink:to="loc_us-gaap_EarningsPerShareDiluted_019f3d79-a01d-7d41-aa09-5a180cb069bb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019f3d79-a01d-7693-a506-07decdd0151c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7af3-adf6-48868c2e0bfb" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019f3d79-a01d-7693-a506-07decdd0151c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019f3d79-a01d-75fe-855c-977f556b1c7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019f3d79-a01d-7693-a506-07decdd0151c" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019f3d79-a01d-75fe-855c-977f556b1c7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019f3d79-a01d-7f30-8a73-5278c44f6c41" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019f3d79-a01d-7693-a506-07decdd0151c" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019f3d79-a01d-7f30-8a73-5278c44f6c41" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITYDEFICIT" xlink:type="simple" xlink:href="toi-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITYDEFICIT"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITYDEFICIT" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019f3d79-a01d-7138-939d-694ab51ef5dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019f3d79-a01d-77ac-a81e-15f5b8a6fcae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7138-939d-694ab51ef5dc" xlink:to="loc_us-gaap_StatementTable_019f3d79-a01d-77ac-a81e-15f5b8a6fcae" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019f3d79-a01d-7189-b6a0-178eb55a72e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019f3d79-a01d-77ac-a81e-15f5b8a6fcae" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019f3d79-a01d-7189-b6a0-178eb55a72e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f3d79-a01d-7189-b6a0-178eb55a72e3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f3d79-a01d-7189-b6a0-178eb55a72e3" xlink:to="loc_us-gaap_EquityComponentDomain_019f3d79-a01d-7189-b6a0-178eb55a72e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019f3d79-a01d-7898-be69-7ce7afcaf326" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019f3d79-a01d-7189-b6a0-178eb55a72e3" xlink:to="loc_us-gaap_EquityComponentDomain_019f3d79-a01d-7898-be69-7ce7afcaf326" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019f3d79-a01d-7bb2-bfde-cd71fbb4aa8a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f3d79-a01d-7898-be69-7ce7afcaf326" xlink:to="loc_us-gaap_CommonStockMember_019f3d79-a01d-7bb2-bfde-cd71fbb4aa8a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockMember_019f3d79-a01d-7910-8a23-c7318c3e7f16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f3d79-a01d-7898-be69-7ce7afcaf326" xlink:to="loc_us-gaap_PreferredStockMember_019f3d79-a01d-7910-8a23-c7318c3e7f16" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019f3d79-a01d-7b90-b539-8a374691e8cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f3d79-a01d-7898-be69-7ce7afcaf326" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019f3d79-a01d-7b90-b539-8a374691e8cf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019f3d79-a01d-7479-a2b7-811995008b3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f3d79-a01d-7898-be69-7ce7afcaf326" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019f3d79-a01d-7479-a2b7-811995008b3d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019f3d79-a01d-7d2e-aafe-06b6ef0543bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019f3d79-a01d-7898-be69-7ce7afcaf326" xlink:to="loc_us-gaap_RetainedEarningsMember_019f3d79-a01d-7d2e-aafe-06b6ef0543bd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019f3d79-a01d-7138-939d-694ab51ef5dc" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019f3d79-a01d-75e7-a14e-e08e6f0d7298" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019f3d79-a01d-75e7-a14e-e08e6f0d7298" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019f3d79-a01d-789c-bf4b-e89e8ddf2fb7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockholdersEquity_019f3d79-a01d-789c-bf4b-e89e8ddf2fb7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019f3d79-a01d-7b1f-bd06-be17002ca5d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_019f3d79-a01d-7b1f-bd06-be17002ca5d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019f3d79-a01d-7251-8363-735f777e3c6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_NetIncomeLoss_019f3d79-a01d-7251-8363-735f777e3c6d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_019f3d79-a01d-76dc-a239-dcb7ea706373" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_019f3d79-a01d-76dc-a239-dcb7ea706373" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019f3d79-a01d-7f6d-a23c-30d91122af1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019f3d79-a01d-7f6d-a23c-30d91122af1c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019f3d79-a01d-7324-b6da-ed4f9880673d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019f3d79-a01d-7324-b6da-ed4f9880673d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019f3d79-a01d-70cb-ad61-42ea34c8de63" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019f3d79-a01d-70cb-ad61-42ea34c8de63" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019f3d79-a01d-7529-98d9-e1011ab4bb9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019f3d79-a01d-7529-98d9-e1011ab4bb9f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfUnits_019f3d79-a01d-73b8-b8e6-c4873d6fcdf4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfUnits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfUnits_019f3d79-a01d-73b8-b8e6-c4873d6fcdf4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfUnits_019f3d79-a01d-78f7-bde4-9bd5dfd4307d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueConversionOfUnits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfUnits_019f3d79-a01d-78f7-bde4-9bd5dfd4307d" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesIssuedForServices_019fb357-1384-743b-8539-c00ca31dbaeb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesIssuedForServices"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesIssuedForServices_019fb357-1384-743b-8539-c00ca31dbaeb" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_019fb357-9a9c-701f-af43-ab9a05ac3627" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_019fb357-9a9c-701f-af43-ab9a05ac3627" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019f3d79-a01d-770e-a146-aa9e7fbb3836" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019f3d79-a01d-770e-a146-aa9e7fbb3836" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019f3d79-a01d-74b0-bed3-cd850b33e032" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019f3d79-a01d-74b0-bed3-cd850b33e032" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019f3d79-a01d-7e3f-ba6e-ac2420c74667" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019f3d79-a01d-724d-85ae-9f3f81dbbf39" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019f3d79-a01d-7e3f-ba6e-ac2420c74667" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019f3d79-a01d-72a0-92a9-04214b004cda" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019f3d79-a01d-79fa-8b62-7d7696592203" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019f3d79-a01d-7840-bcb8-8c7774387b2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesOutstanding"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/SignificantRisksandUncertaintiesIncludingBusinessandCreditConcentrationsSummaryofRevenueConcentrationRiskDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#SignificantRisksandUncertaintiesIncludingBusinessandCreditConcentrationsSummaryofRevenueConcentrationRiskDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/SignificantRisksandUncertaintiesIncludingBusinessandCreditConcentrationsSummaryofRevenueConcentrationRiskDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019f3d79-a0dd-7071-9f71-8b921b1b8fac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7c91-b1d6-e7ee0125a134" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f3d79-a0dd-7071-9f71-8b921b1b8fac" xlink:to="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7c91-b1d6-e7ee0125a134" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019f3d79-a0dd-7f81-b6e8-78b5f9fdaeb0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7c91-b1d6-e7ee0125a134" xlink:to="loc_srt_MajorCustomersAxis_019f3d79-a0dd-7f81-b6e8-78b5f9fdaeb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019f3d79-a0dd-7f81-b6e8-78b5f9fdaeb0_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019f3d79-a0dd-7f81-b6e8-78b5f9fdaeb0" xlink:to="loc_srt_NameOfMajorCustomerDomain_019f3d79-a0dd-7f81-b6e8-78b5f9fdaeb0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019f3d79-a0dd-7c73-8928-2e9136cc9921" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019f3d79-a0dd-7f81-b6e8-78b5f9fdaeb0" xlink:to="loc_srt_NameOfMajorCustomerDomain_019f3d79-a0dd-7c73-8928-2e9136cc9921" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PayorAMember_019f3d79-a0dd-7c27-9537-cd717057dd3a" xlink:href="toi-20260630.xsd#toi_PayorAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f3d79-a0dd-7c73-8928-2e9136cc9921" xlink:to="loc_toi_PayorAMember_019f3d79-a0dd-7c27-9537-cd717057dd3a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PayorCMember_019fa35a-99a2-7834-b8c5-6937458ee6e2" xlink:href="toi-20260630.xsd#toi_PayorCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f3d79-a0dd-7c73-8928-2e9136cc9921" xlink:to="loc_toi_PayorCMember_019fa35a-99a2-7834-b8c5-6937458ee6e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PayorBMember_019f3d79-a0dd-7c5c-942e-2bd3591b71b1" xlink:href="toi-20260630.xsd#toi_PayorBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019f3d79-a0dd-7c73-8928-2e9136cc9921" xlink:to="loc_toi_PayorBMember_019f3d79-a0dd-7c5c-942e-2bd3591b71b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f3d79-a0dd-7c65-ad80-71d4112998a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7c91-b1d6-e7ee0125a134" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f3d79-a0dd-7c65-ad80-71d4112998a3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7c65-ad80-71d4112998a3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f3d79-a0dd-7c65-ad80-71d4112998a3" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7c65-ad80-71d4112998a3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7b59-8478-8f8350f7a0bc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f3d79-a0dd-7c65-ad80-71d4112998a3" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7b59-8478-8f8350f7a0bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerMember_019f3d79-a0dd-79af-a6cc-eee02252ad8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7b59-8478-8f8350f7a0bc" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerMember_019f3d79-a0dd-79af-a6cc-eee02252ad8f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_019f3d79-a0dd-7df5-a041-a5776e511aa5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7b59-8478-8f8350f7a0bc" xlink:to="loc_us-gaap_AccountsReceivableMember_019f3d79-a0dd-7df5-a041-a5776e511aa5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019f3d79-a0dd-7264-9abf-00ca60e79e53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7c91-b1d6-e7ee0125a134" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019f3d79-a0dd-7264-9abf-00ca60e79e53" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7264-9abf-00ca60e79e53_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f3d79-a0dd-7264-9abf-00ca60e79e53" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7264-9abf-00ca60e79e53_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7c98-b839-e4911a27fb99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f3d79-a0dd-7264-9abf-00ca60e79e53" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7c98-b839-e4911a27fb99" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019f3d79-a0dd-775b-9d36-a2e17e535f2b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7c98-b839-e4911a27fb99" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019f3d79-a0dd-775b-9d36-a2e17e535f2b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019f3d79-a0dd-755a-9404-000e35deb32e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f3d79-a0dd-7071-9f71-8b921b1b8fac" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019f3d79-a0dd-755a-9404-000e35deb32e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/SignificantRisksandUncertaintiesIncludingBusinessandCreditConcentrationsSummaryofVendorConcentrationRiskDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#SignificantRisksandUncertaintiesIncludingBusinessandCreditConcentrationsSummaryofVendorConcentrationRiskDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/SignificantRisksandUncertaintiesIncludingBusinessandCreditConcentrationsSummaryofVendorConcentrationRiskDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019f3d79-a0dd-7721-a213-ede29886645b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7894-87c0-72c3179df4db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f3d79-a0dd-7721-a213-ede29886645b" xlink:to="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7894-87c0-72c3179df4db" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f3d79-a0dd-792c-87fa-e5062f9e0ace" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7894-87c0-72c3179df4db" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f3d79-a0dd-792c-87fa-e5062f9e0ace" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-792c-87fa-e5062f9e0ace_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f3d79-a0dd-792c-87fa-e5062f9e0ace" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-792c-87fa-e5062f9e0ace_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7502-88f9-41c9a679724b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019f3d79-a0dd-792c-87fa-e5062f9e0ace" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7502-88f9-41c9a679724b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsTotalMember_019f3d79-a0dd-70b9-8919-414fe98e4045" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsTotalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7502-88f9-41c9a679724b" xlink:to="loc_us-gaap_CostOfGoodsTotalMember_019f3d79-a0dd-70b9-8919-414fe98e4045" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableMember_019f3d79-a0dd-7e4c-a0dd-534a1810e16b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019f3d79-a0dd-7502-88f9-41c9a679724b" xlink:to="loc_us-gaap_AccountsPayableMember_019f3d79-a0dd-7e4c-a0dd-534a1810e16b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019f3d79-a0dd-7dc7-aed1-3c966e5c71fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7894-87c0-72c3179df4db" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019f3d79-a0dd-7dc7-aed1-3c966e5c71fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7dc7-aed1-3c966e5c71fd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f3d79-a0dd-7dc7-aed1-3c966e5c71fd" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7dc7-aed1-3c966e5c71fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7aa6-9035-3c612c1eb1b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019f3d79-a0dd-7dc7-aed1-3c966e5c71fd" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7aa6-9035-3c612c1eb1b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplierConcentrationRiskMember_019f3d79-a0dd-7530-a462-86d92fc80520" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplierConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019f3d79-a0dd-7aa6-9035-3c612c1eb1b4" xlink:to="loc_us-gaap_SupplierConcentrationRiskMember_019f3d79-a0dd-7530-a462-86d92fc80520" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f3d79-a0dd-7458-ab91-b6c3bb24cdab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019f3d79-a0dd-7894-87c0-72c3179df4db" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f3d79-a0dd-7458-ab91-b6c3bb24cdab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f3d79-a0dd-7458-ab91-b6c3bb24cdab_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f3d79-a0dd-7458-ab91-b6c3bb24cdab" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f3d79-a0dd-7458-ab91-b6c3bb24cdab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f3d79-a0dd-7b15-9098-d67bf9dd830c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_019f3d79-a0dd-7458-ab91-b6c3bb24cdab" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f3d79-a0dd-7b15-9098-d67bf9dd830c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_VendorAMember_019f3d79-a0dd-7bd6-8658-6e8e064a7799" xlink:href="toi-20260630.xsd#toi_VendorAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f3d79-a0dd-7b15-9098-d67bf9dd830c" xlink:to="loc_toi_VendorAMember_019f3d79-a0dd-7bd6-8658-6e8e064a7799" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_VendorBMember_019f3d79-a0dd-7649-99c7-2a4a597f6772" xlink:href="toi-20260630.xsd#toi_VendorBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_019f3d79-a0dd-7b15-9098-d67bf9dd830c" xlink:to="loc_toi_VendorBMember_019f3d79-a0dd-7649-99c7-2a4a597f6772" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019f3d79-a0dd-7754-8596-6ff31125f048" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019f3d79-a0dd-7721-a213-ede29886645b" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019f3d79-a0dd-7754-8596-6ff31125f048" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/AccountsReceivableScheduleofAccountsReceivableDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#AccountsReceivableScheduleofAccountsReceivableDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/AccountsReceivableScheduleofAccountsReceivableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019f3d79-a0dd-7dd6-83a1-f9b0da4376ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsNotesAndLoansReceivableLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_019f3d79-a0dd-76bc-9adc-88c4ddb12da5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019f3d79-a0dd-7dd6-83a1-f9b0da4376ce" xlink:to="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_019f3d79-a0dd-76bc-9adc-88c4ddb12da5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019f3d79-a0dd-71f6-8b3e-87bc26a37b95" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_019f3d79-a0dd-76bc-9adc-88c4ddb12da5" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019f3d79-a0dd-71f6-8b3e-87bc26a37b95" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019f3d79-a0dd-71f6-8b3e-87bc26a37b95_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019f3d79-a0dd-71f6-8b3e-87bc26a37b95" xlink:to="loc_us-gaap_ReceivableTypeDomain_019f3d79-a0dd-71f6-8b3e-87bc26a37b95_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivableTypeDomain_019f3d79-a0dd-7b53-9fca-c7b6988b74b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivableTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis_019f3d79-a0dd-71f6-8b3e-87bc26a37b95" xlink:to="loc_us-gaap_ReceivableTypeDomain_019f3d79-a0dd-7b53-9fca-c7b6988b74b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_OralDrugAccountsReceivableMember_019f3d79-a0dd-7cf8-80a8-a8fbd1249042" xlink:href="toi-20260630.xsd#toi_OralDrugAccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_019f3d79-a0dd-7b53-9fca-c7b6988b74b2" xlink:to="loc_toi_OralDrugAccountsReceivableMember_019f3d79-a0dd-7cf8-80a8-a8fbd1249042" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CapitatedAccountsReceivableMember_019f3d79-a0dd-7c71-80e3-036f24d330e7" xlink:href="toi-20260630.xsd#toi_CapitatedAccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_019f3d79-a0dd-7b53-9fca-c7b6988b74b2" xlink:to="loc_toi_CapitatedAccountsReceivableMember_019f3d79-a0dd-7c71-80e3-036f24d330e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_FfsAccountsReceivableMember_019f3d79-a0dd-7cb2-8bae-b5ab194596c0" xlink:href="toi-20260630.xsd#toi_FfsAccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_019f3d79-a0dd-7b53-9fca-c7b6988b74b2" xlink:to="loc_toi_FfsAccountsReceivableMember_019f3d79-a0dd-7cb2-8bae-b5ab194596c0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_OtherTradeReceivableMember_019f3d79-a0dd-73a3-85d0-a1a9a8db9ea9" xlink:href="toi-20260630.xsd#toi_OtherTradeReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivableTypeDomain_019f3d79-a0dd-7b53-9fca-c7b6988b74b2" xlink:to="loc_toi_OtherTradeReceivableMember_019f3d79-a0dd-73a3-85d0-a1a9a8db9ea9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019f3d79-a0dd-78d9-8f3c-608a9973d5a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019f3d79-a0dd-7dd6-83a1-f9b0da4376ce" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019f3d79-a0dd-78d9-8f3c-608a9973d5a4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/RevenueSummaryofDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#RevenueSummaryofDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/RevenueSummaryofDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019f3d79-a0dd-7ce9-8ba8-e04eb1493f09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019f3d79-a0dd-7f69-96ce-a4e1bef27c72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019f3d79-a0dd-7ce9-8ba8-e04eb1493f09" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019f3d79-a0dd-7f69-96ce-a4e1bef27c72" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019f3d79-a0dd-7068-9c0b-1e5a49727b06" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019f3d79-a0dd-7f69-96ce-a4e1bef27c72" xlink:to="loc_srt_ProductOrServiceAxis_019f3d79-a0dd-7068-9c0b-1e5a49727b06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f3d79-a0dd-7068-9c0b-1e5a49727b06_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019f3d79-a0dd-7068-9c0b-1e5a49727b06" xlink:to="loc_srt_ProductsAndServicesDomain_019f3d79-a0dd-7068-9c0b-1e5a49727b06_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f3d79-a0dd-7597-a779-6c4c2686b754" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019f3d79-a0dd-7068-9c0b-1e5a49727b06" xlink:to="loc_srt_ProductsAndServicesDomain_019f3d79-a0dd-7597-a779-6c4c2686b754" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HealthCarePatientServiceMember_019f3d79-a0dd-737c-abee-1df679f00b46" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HealthCarePatientServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a0dd-7597-a779-6c4c2686b754" xlink:to="loc_us-gaap_HealthCarePatientServiceMember_019f3d79-a0dd-737c-abee-1df679f00b46" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CapitatedRevenueMember_019f3d79-a0dd-7f24-bd40-1bf3e1caa7ea" xlink:href="toi-20260630.xsd#toi_CapitatedRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HealthCarePatientServiceMember_019f3d79-a0dd-737c-abee-1df679f00b46" xlink:to="loc_toi_CapitatedRevenueMember_019f3d79-a0dd-7f24-bd40-1bf3e1caa7ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_FeeForServiceMember_019f3d79-a0dd-7c4d-a61d-bcadac61583c" xlink:href="toi-20260630.xsd#toi_FeeForServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HealthCarePatientServiceMember_019f3d79-a0dd-737c-abee-1df679f00b46" xlink:to="loc_toi_FeeForServiceMember_019f3d79-a0dd-7c4d-a61d-bcadac61583c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_HealthCareSpecialtyPharmacyMember_019f3d79-a0dd-710f-b8d4-ea72465e370d" xlink:href="toi-20260630.xsd#toi_HealthCareSpecialtyPharmacyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a0dd-7597-a779-6c4c2686b754" xlink:to="loc_toi_HealthCareSpecialtyPharmacyMember_019f3d79-a0dd-710f-b8d4-ea72465e370d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClinicalResearchTrialsAndOtherRevenueMember_019f3d79-a0dd-7438-894c-c5c66d46e545" xlink:href="toi-20260630.xsd#toi_ClinicalResearchTrialsAndOtherRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a0dd-7597-a779-6c4c2686b754" xlink:to="loc_toi_ClinicalResearchTrialsAndOtherRevenueMember_019f3d79-a0dd-7438-894c-c5c66d46e545" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019f3d79-a0dd-7278-850e-9fd1810d666f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019f3d79-a0dd-7ce9-8ba8-e04eb1493f09" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019f3d79-a0dd-7278-850e-9fd1810d666f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/InventoriesDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#InventoriesDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/InventoriesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryLineItems_019f3d79-a0dd-7fef-8f77-5449ca6f40be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryCurrentTable_019f3d79-a0dd-713f-a81a-9b4a41b4e654" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryCurrentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_InventoryLineItems_019f3d79-a0dd-7fef-8f77-5449ca6f40be" xlink:to="loc_us-gaap_InventoryCurrentTable_019f3d79-a0dd-713f-a81a-9b4a41b4e654" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesInventoryAxis_019f3d79-a0dd-7808-a864-267b5e9dc51a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesInventoryAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_InventoryCurrentTable_019f3d79-a0dd-713f-a81a-9b4a41b4e654" xlink:to="loc_us-gaap_PublicUtilitiesInventoryAxis_019f3d79-a0dd-7808-a864-267b5e9dc51a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_019f3d79-a0dd-7808-a864-267b5e9dc51a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesInventoryTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PublicUtilitiesInventoryAxis_019f3d79-a0dd-7808-a864-267b5e9dc51a" xlink:to="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_019f3d79-a0dd-7808-a864-267b5e9dc51a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_019f3d79-a0dd-734f-b3d6-447aee4de546" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesInventoryTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PublicUtilitiesInventoryAxis_019f3d79-a0dd-7808-a864-267b5e9dc51a" xlink:to="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_019f3d79-a0dd-734f-b3d6-447aee4de546" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_OralDrugInventoryMember_019f3d79-a0dd-7878-9c89-9b17bd3b1324" xlink:href="toi-20260630.xsd#toi_OralDrugInventoryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_019f3d79-a0dd-734f-b3d6-447aee4de546" xlink:to="loc_toi_OralDrugInventoryMember_019f3d79-a0dd-7878-9c89-9b17bd3b1324" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_IvDrugInventoryMember_019f3d79-a0dd-715b-bbcf-124aeffee81d" xlink:href="toi-20260630.xsd#toi_IvDrugInventoryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesInventoryTypeDomain_019f3d79-a0dd-734f-b3d6-447aee4de546" xlink:to="loc_toi_IvDrugInventoryMember_019f3d79-a0dd-715b-bbcf-124aeffee81d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_019f3d79-a0dd-736f-91a6-53f804b38150" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_019f3d79-a0dd-7fef-8f77-5449ca6f40be" xlink:to="loc_us-gaap_InventoryNet_019f3d79-a0dd-736f-91a6-53f804b38150" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/MarketableSecuritiesandFairValueMeasurementsSummaryofInvestmentSecuritiesClassifiedasAvailableforsaleDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#MarketableSecuritiesandFairValueMeasurementsSummaryofInvestmentSecuritiesClassifiedasAvailableforsaleDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/MarketableSecuritiesandFairValueMeasurementsSummaryofInvestmentSecuritiesClassifiedasAvailableforsaleDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-7f37-a07a-f673732643f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f3d79-a0dd-7b09-9103-78ec75c0670c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-7f37-a07a-f673732643f4" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f3d79-a0dd-7b09-9103-78ec75c0670c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019f457d-993d-7d45-95b5-a5b2a46ca192" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f3d79-a0dd-7b09-9103-78ec75c0670c" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019f457d-993d-7d45-95b5-a5b2a46ca192" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f457d-993d-7d45-95b5-a5b2a46ca192_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f457d-993d-7d45-95b5-a5b2a46ca192" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f457d-993d-7d45-95b5-a5b2a46ca192_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f457d-993d-75fc-bfa3-9a3a033df4e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019f457d-993d-7d45-95b5-a5b2a46ca192" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f457d-993d-75fc-bfa3-9a3a033df4e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019f457d-cd5f-703f-bfcc-a89f7763948c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019f457d-993d-75fc-bfa3-9a3a033df4e7" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019f457d-cd5f-703f-bfcc-a89f7763948c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss_019f3d79-a0dd-78e8-beb0-6af7c2f5ea60" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-7f37-a07a-f673732643f4" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss_019f3d79-a0dd-78e8-beb0-6af7c2f5ea60" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019f3d79-a0dd-70ea-8caf-f84a6c4981b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-7f37-a07a-f673732643f4" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019f3d79-a0dd-70ea-8caf-f84a6c4981b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019f3d79-a0dd-7222-8381-565a4cbfa1e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-7f37-a07a-f673732643f4" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019f3d79-a0dd-7222-8381-565a4cbfa1e4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019f3d79-a0dd-7a83-aea9-272bd095bc25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-7f37-a07a-f673732643f4" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019f3d79-a0dd-7a83-aea9-272bd095bc25" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f3d79-a0dd-79d8-89dd-f40e29483197" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-7f37-a07a-f673732643f4" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f3d79-a0dd-79d8-89dd-f40e29483197" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_019f3d79-a0dd-760f-b126-66a3e99266fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f3d79-a0dd-79d8-89dd-f40e29483197" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost_019f3d79-a0dd-760f-b126-66a3e99266fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_019f3d79-a0dd-7f30-8f49-72072143250b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f3d79-a0dd-79d8-89dd-f40e29483197" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost_019f3d79-a0dd-7f30-8f49-72072143250b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtSecuritiesAvailableForSaleAmortizedCostMaturityAllocatedAndSingleMaturityDateAfterYear5_019f3d79-a0dd-71bd-b86c-0d27991d6263" xlink:href="toi-20260630.xsd#toi_DebtSecuritiesAvailableForSaleAmortizedCostMaturityAllocatedAndSingleMaturityDateAfterYear5"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f3d79-a0dd-79d8-89dd-f40e29483197" xlink:to="loc_toi_DebtSecuritiesAvailableForSaleAmortizedCostMaturityAllocatedAndSingleMaturityDateAfterYear5_019f3d79-a0dd-71bd-b86c-0d27991d6263" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis_019f3d79-a0dd-7485-bd0d-823d3f251d60" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasisAbstract_019f3d79-a0dd-79d8-89dd-f40e29483197" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesSingleMaturityDateAmortizedCostBasis_019f3d79-a0dd-7485-bd0d-823d3f251d60" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/MarketableSecuritiesandFairValueMeasurementsSummaryofCarryingAmountsofFinancialInstrumentsDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#MarketableSecuritiesandFairValueMeasurementsSummaryofCarryingAmountsofFinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/MarketableSecuritiesandFairValueMeasurementsSummaryofCarryingAmountsofFinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_019f3d79-a0dd-7e01-b844-c213a9bdb0f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019f3d79-a0dd-7539-a531-39a676772ca9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019f3d79-a0dd-7e01-b844-c213a9bdb0f1" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019f3d79-a0dd-7539-a531-39a676772ca9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f3d79-a0dd-76e0-8077-c4196edeca36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_019f3d79-a0dd-7539-a531-39a676772ca9" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f3d79-a0dd-76e0-8077-c4196edeca36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-76e0-8077-c4196edeca36_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f3d79-a0dd-76e0-8077-c4196edeca36" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-76e0-8077-c4196edeca36_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7aef-bc22-0f71c7a11188" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f3d79-a0dd-76e0-8077-c4196edeca36" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7aef-bc22-0f71c7a11188" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019f3d79-a0dd-7e6a-9fb7-fe2e05176b57" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7aef-bc22-0f71c7a11188" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019f3d79-a0dd-7e6a-9fb7-fe2e05176b57" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019f3d79-a0dd-7b90-a416-4c914a4c9838" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7aef-bc22-0f71c7a11188" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019f3d79-a0dd-7b90-a416-4c914a4c9838" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019f3d79-a0dd-7019-9bd3-573cb0fee0a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7aef-bc22-0f71c7a11188" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019f3d79-a0dd-7019-9bd3-573cb0fee0a9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019f3d79-a0dd-711e-8736-cd16d6f729c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019f3d79-a0dd-7e01-b844-c213a9bdb0f1" xlink:to="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019f3d79-a0dd-711e-8736-cd16d6f729c6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019f3d79-a0dd-7aaf-bed1-2506ab2649ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_019f3d79-a0dd-711e-8736-cd16d6f729c6" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019f3d79-a0dd-7aaf-bed1-2506ab2649ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019f3d79-a0dd-7bb5-9fb7-180fcd8e17fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_019f3d79-a0dd-7e01-b844-c213a9bdb0f1" xlink:to="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019f3d79-a0dd-7bb5-9fb7-180fcd8e17fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DerivativeWarrantLiabilitiesNoncurrent_019f3d79-a0dd-7ae4-9148-285ab9c74dc6" xlink:href="toi-20260630.xsd#toi_DerivativeWarrantLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019f3d79-a0dd-7bb5-9fb7-180fcd8e17fa" xlink:to="loc_toi_DerivativeWarrantLiabilitiesNoncurrent_019f3d79-a0dd-7ae4-9148-285ab9c74dc6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilitiesNoncurrent_019f3d79-a0dd-7db5-9017-d6e67d637e61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_019f3d79-a0dd-7bb5-9fb7-180fcd8e17fa" xlink:to="loc_us-gaap_DerivativeLiabilitiesNoncurrent_019f3d79-a0dd-7db5-9017-d6e67d637e61" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/MarketableSecuritiesandFairValueMeasurementsSummaryofChangesinFairValueofLevel3WarrantLiabilitiesDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#MarketableSecuritiesandFairValueMeasurementsSummaryofChangesinFairValueofLevel3WarrantLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/MarketableSecuritiesandFairValueMeasurementsSummaryofChangesinFairValueofLevel3WarrantLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-794f-8c3d-66a9179ad1f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f3d79-a0dd-7828-a702-b8d5fe2dcf09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-794f-8c3d-66a9179ad1f2" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f3d79-a0dd-7828-a702-b8d5fe2dcf09" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_019f3d79-a0dd-7273-a5b0-13cab118918b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019f3d79-a0dd-7828-a702-b8d5fe2dcf09" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_019f3d79-a0dd-7273-a5b0-13cab118918b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f3d79-a0dd-7273-a5b0-13cab118918b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019f3d79-a0dd-7273-a5b0-13cab118918b" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f3d79-a0dd-7273-a5b0-13cab118918b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f3d79-a0dd-7198-925b-992565036314" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_019f3d79-a0dd-7273-a5b0-13cab118918b" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f3d79-a0dd-7198-925b-992565036314" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFinancialInstrumentsLiabilitiesMember_019f3d79-a0dd-7c2a-a001-0f60d985fcd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeFinancialInstrumentsLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_019f3d79-a0dd-7198-925b-992565036314" xlink:to="loc_us-gaap_DerivativeFinancialInstrumentsLiabilitiesMember_019f3d79-a0dd-7c2a-a001-0f60d985fcd8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f3d79-a0dd-76d8-bd48-8aea25cbe4bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019f3d79-a0dd-794f-8c3d-66a9179ad1f2" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f3d79-a0dd-76d8-bd48-8aea25cbe4bf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019f3d79-a0dd-752d-b087-ef50a9dc0e53" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f3d79-a0dd-76d8-bd48-8aea25cbe4bf" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019f3d79-a0dd-752d-b087-ef50a9dc0e53" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_019f3d79-a0dd-795b-8aa2-3c68608af1b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019f3d79-a0dd-76d8-bd48-8aea25cbe4bf" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_019f3d79-a0dd-795b-8aa2-3c68608af1b4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019f3d79-a0dd-7dad-8063-27f38a2396ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/MarketableSecuritiesandFairValueMeasurementsSummaryofAssumptionsusedintheValuationofDerivativeLiabilitiesDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#MarketableSecuritiesandFairValueMeasurementsSummaryofAssumptionsusedintheValuationofDerivativeLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/MarketableSecuritiesandFairValueMeasurementsSummaryofAssumptionsusedintheValuationofDerivativeLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019f3d79-a0dd-7411-b99c-ad0da482a956" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019f3d79-a0dd-7a2b-a76b-404f00d53246" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019f3d79-a0dd-7411-b99c-ad0da482a956" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019f3d79-a0dd-7a2b-a76b-404f00d53246" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_019f3d79-a0dd-7d3c-b5ef-3c9a218ae821" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019f3d79-a0dd-7a2b-a76b-404f00d53246" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_019f3d79-a0dd-7d3c-b5ef-3c9a218ae821" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7d3c-b5ef-3c9a218ae821_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_019f3d79-a0dd-7d3c-b5ef-3c9a218ae821" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7d3c-b5ef-3c9a218ae821_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7b97-8c28-c174c3a3e055" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_019f3d79-a0dd-7d3c-b5ef-3c9a218ae821" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7b97-8c28-c174c3a3e055" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_MeasurementInputUnitPriceMember_019f3d79-a0dd-7727-bef4-6da3907a86e1" xlink:href="toi-20260630.xsd#toi_MeasurementInputUnitPriceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7b97-8c28-c174c3a3e055" xlink:to="loc_toi_MeasurementInputUnitPriceMember_019f3d79-a0dd-7727-bef4-6da3907a86e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_019f3d79-a0dd-748c-94c5-8189f9e02d84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputExpectedTermMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7b97-8c28-c174c3a3e055" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_019f3d79-a0dd-748c-94c5-8189f9e02d84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_019f3d79-a0dd-7bca-8fcb-04c3b4d3f970" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputPriceVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7b97-8c28-c174c3a3e055" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_019f3d79-a0dd-7bca-8fcb-04c3b4d3f970" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_019f3d79-a0dd-7e41-86c5-96d414ed9bc5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7b97-8c28-c174c3a3e055" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_019f3d79-a0dd-7e41-86c5-96d414ed9bc5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_019f3d79-a0dd-7705-8463-d519262c040e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputExpectedDividendRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7b97-8c28-c174c3a3e055" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_019f3d79-a0dd-7705-8463-d519262c040e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_MeasurementInputCostOfEquityMember_019f3d79-a0dd-7826-b2dc-079c0c2fd428" xlink:href="toi-20260630.xsd#toi_MeasurementInputCostOfEquityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019f3d79-a0dd-7b97-8c28-c174c3a3e055" xlink:to="loc_toi_MeasurementInputCostOfEquityMember_019f3d79-a0dd-7826-b2dc-079c0c2fd428" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019f3d79-a0dd-7590-825d-7eb556fab986" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019f3d79-a0dd-7a2b-a76b-404f00d53246" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019f3d79-a0dd-7590-825d-7eb556fab986" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019f3d79-a0dd-7590-825d-7eb556fab986_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019f3d79-a0dd-7590-825d-7eb556fab986" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019f3d79-a0dd-7590-825d-7eb556fab986_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019f3d79-a0dd-7c0e-b1e2-3b19112bcb64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019f3d79-a0dd-7590-825d-7eb556fab986" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019f3d79-a0dd-7c0e-b1e2-3b19112bcb64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DerivativeInstrumentMember_019f3d79-a0dd-78aa-b8f7-fd9d805ab649" xlink:href="toi-20260630.xsd#toi_DerivativeInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019f3d79-a0dd-7c0e-b1e2-3b19112bcb64" xlink:to="loc_toi_DerivativeInstrumentMember_019f3d79-a0dd-78aa-b8f7-fd9d805ab649" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilityMeasurementInput_019f3d79-a0dd-7971-90de-0b87b1ca6d4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLiabilityMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019f3d79-a0dd-7411-b99c-ad0da482a956" xlink:to="loc_us-gaap_DerivativeLiabilityMeasurementInput_019f3d79-a0dd-7971-90de-0b87b1ca6d4f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/PropertyandEquipmentNetSummaryofPropertyandEquipmentNetDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#PropertyandEquipmentNetSummaryofPropertyandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/PropertyandEquipmentNetSummaryofPropertyandEquipmentNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f3d79-a0dd-7c7c-b1e7-ed44c424bbfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019f3d79-a0dd-7654-81f4-163cefe72274" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f3d79-a0dd-7c7c-b1e7-ed44c424bbfe" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019f3d79-a0dd-7654-81f4-163cefe72274" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019f3d79-a0dd-702f-807f-b44243f35db7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019f3d79-a0dd-7654-81f4-163cefe72274" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019f3d79-a0dd-702f-807f-b44243f35db7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-702f-807f-b44243f35db7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019f3d79-a0dd-702f-807f-b44243f35db7" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-702f-807f-b44243f35db7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-7044-80f8-dd870177af05" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019f3d79-a0dd-702f-807f-b44243f35db7" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-7044-80f8-dd870177af05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ComputerAndSoftwareMember_019f3d79-a0dd-76cd-9b99-947d1d660d22" xlink:href="toi-20260630.xsd#toi_ComputerAndSoftwareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-7044-80f8-dd870177af05" xlink:to="loc_toi_ComputerAndSoftwareMember_019f3d79-a0dd-76cd-9b99-947d1d660d22" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_019f3d79-a0dd-740d-a326-c36bccbaa48c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-7044-80f8-dd870177af05" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_019f3d79-a0dd-740d-a326-c36bccbaa48c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_019f3d79-a0dd-7146-88f6-1133597be75c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-7044-80f8-dd870177af05" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_019f3d79-a0dd-7146-88f6-1133597be75c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquipmentMember_019f3d79-a0dd-7538-85a2-538ff0a5ec11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-7044-80f8-dd870177af05" xlink:to="loc_us-gaap_EquipmentMember_019f3d79-a0dd-7538-85a2-538ff0a5ec11" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_019f3d79-a0dd-7adf-8ab6-509f190e7aef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-7044-80f8-dd870177af05" xlink:to="loc_us-gaap_ConstructionInProgressMember_019f3d79-a0dd-7adf-8ab6-509f190e7aef" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_FinanceLeaseROUAssetsMember_019f3d79-a0dd-7449-b7ca-78ff80fb6c1b" xlink:href="toi-20260630.xsd#toi_FinanceLeaseROUAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019f3d79-a0dd-7044-80f8-dd870177af05" xlink:to="loc_toi_FinanceLeaseROUAssetsMember_019f3d79-a0dd-7449-b7ca-78ff80fb6c1b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019f3d79-a0dd-70d1-b511-46a91ecd9b58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f3d79-a0dd-7c7c-b1e7-ed44c424bbfe" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019f3d79-a0dd-70d1-b511-46a91ecd9b58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019f3d79-a0dd-7d64-9738-d88897f37f25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f3d79-a0dd-7c7c-b1e7-ed44c424bbfe" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019f3d79-a0dd-7d64-9738-d88897f37f25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019f3d79-a0dd-7fdc-8390-16379c0e7ebe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f3d79-a0dd-7c7c-b1e7-ed44c424bbfe" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019f3d79-a0dd-7fdc-8390-16379c0e7ebe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019f3d79-a0dd-74dd-95fe-795708c637ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019f3d79-a0dd-7c7c-b1e7-ed44c424bbfe" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019f3d79-a0dd-74dd-95fe-795708c637ad" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/AccruedExpensesandOtherCurrentLiabilitiesAdditionalInformationDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#AccruedExpensesandOtherCurrentLiabilitiesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/AccruedExpensesandOtherCurrentLiabilitiesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplierFinanceProgramLineItems_019f3d79-a0dd-7136-b231-3a24dc65c00d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplierFinanceProgramLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplierFinanceProgramTable_019f3d79-a0dd-7735-89e6-21f3974bd86c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplierFinanceProgramTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SupplierFinanceProgramLineItems_019f3d79-a0dd-7136-b231-3a24dc65c00d" xlink:to="loc_us-gaap_SupplierFinanceProgramTable_019f3d79-a0dd-7735-89e6-21f3974bd86c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f3d79-a0dd-7761-b6e9-6ea345057d2b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SupplierFinanceProgramTable_019f3d79-a0dd-7735-89e6-21f3974bd86c" xlink:to="loc_srt_RangeAxis_019f3d79-a0dd-7761-b6e9-6ea345057d2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f3d79-a0dd-7761-b6e9-6ea345057d2b_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f3d79-a0dd-7761-b6e9-6ea345057d2b" xlink:to="loc_srt_RangeMember_019f3d79-a0dd-7761-b6e9-6ea345057d2b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f3d79-a0dd-7d79-9c45-5947ad97e079" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f3d79-a0dd-7761-b6e9-6ea345057d2b" xlink:to="loc_srt_RangeMember_019f3d79-a0dd-7d79-9c45-5947ad97e079" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f3d79-a0dd-710b-b5e1-99628d908934" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f3d79-a0dd-7d79-9c45-5947ad97e079" xlink:to="loc_srt_MinimumMember_019f3d79-a0dd-710b-b5e1-99628d908934" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f3d79-a0dd-7e21-a923-3e71005acbfb" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f3d79-a0dd-7d79-9c45-5947ad97e079" xlink:to="loc_srt_MaximumMember_019f3d79-a0dd-7e21-a923-3e71005acbfb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrentAndNoncurrent_019f3d79-a0dd-7926-87fd-f74499331f6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplierFinanceProgramLineItems_019f3d79-a0dd-7136-b231-3a24dc65c00d" xlink:to="loc_us-gaap_AccountsPayableCurrentAndNoncurrent_019f3d79-a0dd-7926-87fd-f74499331f6f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableInterestBearingInterestRate_019f3d79-a0dd-7d2c-8db4-3b5c3307969d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableInterestBearingInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplierFinanceProgramLineItems_019f3d79-a0dd-7136-b231-3a24dc65c00d" xlink:to="loc_us-gaap_AccountsPayableInterestBearingInterestRate_019f3d79-a0dd-7d2c-8db4-3b5c3307969d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_NumberOfInstalmentsPayable_019f3d79-a0dd-746d-92a2-899812842f6e" xlink:href="toi-20260630.xsd#toi_NumberOfInstalmentsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplierFinanceProgramLineItems_019f3d79-a0dd-7136-b231-3a24dc65c00d" xlink:to="loc_toi_NumberOfInstalmentsPayable_019f3d79-a0dd-746d-92a2-899812842f6e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedInsuranceCurrent_019f3d79-a0dd-7fc6-85df-4cc4963b3b54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedInsuranceCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplierFinanceProgramLineItems_019f3d79-a0dd-7136-b231-3a24dc65c00d" xlink:to="loc_us-gaap_AccruedInsuranceCurrent_019f3d79-a0dd-7fc6-85df-4cc4963b3b54" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedInsuranceCurrentAndNoncurrent_019f3d79-a0dd-76cb-95a4-d2d3ca74280e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedInsuranceCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplierFinanceProgramLineItems_019f3d79-a0dd-7136-b231-3a24dc65c00d" xlink:to="loc_us-gaap_AccruedInsuranceCurrentAndNoncurrent_019f3d79-a0dd-76cb-95a4-d2d3ca74280e" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/LeasesAdditionalInformationDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#LeasesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/LeasesAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f3d79-a0dd-72e9-bdaa-96354f54e9ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_019f3d79-a0dd-703b-95ce-b2d3eb67d447" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f3d79-a0dd-72e9-bdaa-96354f54e9ac" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_019f3d79-a0dd-703b-95ce-b2d3eb67d447" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f3d79-a0dd-7084-ad5c-f05be3ebd539" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019f3d79-a0dd-703b-95ce-b2d3eb67d447" xlink:to="loc_srt_RangeAxis_019f3d79-a0dd-7084-ad5c-f05be3ebd539" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f3d79-a0dd-7084-ad5c-f05be3ebd539_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f3d79-a0dd-7084-ad5c-f05be3ebd539" xlink:to="loc_srt_RangeMember_019f3d79-a0dd-7084-ad5c-f05be3ebd539_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f3d79-a0dd-7945-a18e-ce23d26f45b8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f3d79-a0dd-7084-ad5c-f05be3ebd539" xlink:to="loc_srt_RangeMember_019f3d79-a0dd-7945-a18e-ce23d26f45b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f3d79-a0dd-7c44-9378-c43aef7ac137" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f3d79-a0dd-7945-a18e-ce23d26f45b8" xlink:to="loc_srt_MinimumMember_019f3d79-a0dd-7c44-9378-c43aef7ac137" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f3d79-a0dd-7389-a225-2bf92be431b5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f3d79-a0dd-7945-a18e-ce23d26f45b8" xlink:to="loc_srt_MaximumMember_019f3d79-a0dd-7389-a225-2bf92be431b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019f3d79-a0dd-7c33-8f24-18f4f1fc120b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019f3d79-a0dd-703b-95ce-b2d3eb67d447" xlink:to="loc_srt_StatementGeographicalAxis_019f3d79-a0dd-7c33-8f24-18f4f1fc120b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019f3d79-a0dd-7c33-8f24-18f4f1fc120b_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019f3d79-a0dd-7c33-8f24-18f4f1fc120b" xlink:to="loc_srt_SegmentGeographicalDomain_019f3d79-a0dd-7c33-8f24-18f4f1fc120b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019f3d79-a0dd-791c-8c17-d40a2fdd3d0f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019f3d79-a0dd-7c33-8f24-18f4f1fc120b" xlink:to="loc_srt_SegmentGeographicalDomain_019f3d79-a0dd-791c-8c17-d40a2fdd3d0f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_FL_019f3d79-a0dd-7761-ba0a-73f941394e75" xlink:href="https://xbrl.sec.gov/stpr/2026/stpr-2026.xsd#stpr_FL"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019f3d79-a0dd-791c-8c17-d40a2fdd3d0f" xlink:to="loc_stpr_FL_019f3d79-a0dd-7761-ba0a-73f941394e75" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_CA_019f3d79-a0dd-7578-8027-a6fe851ffb1e" xlink:href="https://xbrl.sec.gov/stpr/2026/stpr-2026.xsd#stpr_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019f3d79-a0dd-791c-8c17-d40a2fdd3d0f" xlink:to="loc_stpr_CA_019f3d79-a0dd-7578-8027-a6fe851ffb1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019f3d79-a0dd-79a5-80b8-5eb86e8310b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f3d79-a0dd-72e9-bdaa-96354f54e9ac" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019f3d79-a0dd-79a5-80b8-5eb86e8310b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasePayments_019f3d79-a0dd-76d8-b682-a717dfd6030c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeasePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f3d79-a0dd-72e9-bdaa-96354f54e9ac" xlink:to="loc_us-gaap_OperatingLeasePayments_019f3d79-a0dd-76d8-b682-a717dfd6030c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_LesseeOperatingLeaseNumberOfLeasesExtended_019f3d79-a0dd-79c6-958f-0b41292d75ba" xlink:href="toi-20260630.xsd#toi_LesseeOperatingLeaseNumberOfLeasesExtended"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f3d79-a0dd-72e9-bdaa-96354f54e9ac" xlink:to="loc_toi_LesseeOperatingLeaseNumberOfLeasesExtended_019f3d79-a0dd-79c6-958f-0b41292d75ba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_LesseeOperatingLeaseIncreaseInLiabilityAndRightOfUseAsset_019f3d79-a0dd-7069-9c9c-2621ceb9baf1" xlink:href="toi-20260630.xsd#toi_LesseeOperatingLeaseIncreaseInLiabilityAndRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019f3d79-a0dd-72e9-bdaa-96354f54e9ac" xlink:to="loc_toi_LesseeOperatingLeaseIncreaseInLiabilityAndRightOfUseAsset_019f3d79-a0dd-7069-9c9c-2621ceb9baf1" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/DebtAdditionalInformationDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#DebtAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/DebtAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-70b1-8247-d30c63d515b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-70b1-8247-d30c63d515b7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019f3d79-a0dd-736e-91c3-ebd9770906ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-70b1-8247-d30c63d515b7" xlink:to="loc_us-gaap_DebtInstrumentAxis_019f3d79-a0dd-736e-91c3-ebd9770906ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-736e-91c3-ebd9770906ed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f3d79-a0dd-736e-91c3-ebd9770906ed" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-736e-91c3-ebd9770906ed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-719f-a133-c8b904ddb9d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f3d79-a0dd-736e-91c3-ebd9770906ed" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-719f-a133-c8b904ddb9d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_FacilityAgreementMember_019f3d79-a0dd-797f-93e2-f61d6c438c63" xlink:href="toi-20260630.xsd#toi_FacilityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-719f-a133-c8b904ddb9d9" xlink:to="loc_toi_FacilityAgreementMember_019f3d79-a0dd-797f-93e2-f61d6c438c63" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019f3d79-a0dd-7e61-94b8-3642ad7ee08d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-70b1-8247-d30c63d515b7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019f3d79-a0dd-7e61-94b8-3642ad7ee08d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-7e61-94b8-3642ad7ee08d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f3d79-a0dd-7e61-94b8-3642ad7ee08d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-7e61-94b8-3642ad7ee08d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-759c-824a-0c75388911c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f3d79-a0dd-7e61-94b8-3642ad7ee08d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-759c-824a-0c75388911c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019f3d79-a0dd-70bd-8dbd-0c3920f66d45" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-759c-824a-0c75388911c2" xlink:to="loc_us-gaap_ConvertibleDebtMember_019f3d79-a0dd-70bd-8dbd-0c3920f66d45" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ConvertibleNoteWarrantMember_019f3d79-a0dd-79b2-9a3f-732a5cb1fcf5" xlink:href="toi-20260630.xsd#toi_ConvertibleNoteWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-759c-824a-0c75388911c2" xlink:to="loc_toi_ConvertibleNoteWarrantMember_019f3d79-a0dd-79b2-9a3f-732a5cb1fcf5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_OptionalRedemptionMember_019f3d79-a0dd-7d94-9154-abdd887c5828" xlink:href="toi-20260630.xsd#toi_OptionalRedemptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-759c-824a-0c75388911c2" xlink:to="loc_toi_OptionalRedemptionMember_019f3d79-a0dd-7d94-9154-abdd887c5828" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_EmbeddedConversionOptionFeatureMember_019f3d79-a0dd-7638-b1f3-1ca377f41de0" xlink:href="toi-20260630.xsd#toi_EmbeddedConversionOptionFeatureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-759c-824a-0c75388911c2" xlink:to="loc_toi_EmbeddedConversionOptionFeatureMember_019f3d79-a0dd-7638-b1f3-1ca377f41de0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f3d79-a0dd-769b-807a-69f8dc9f64ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-70b1-8247-d30c63d515b7" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f3d79-a0dd-769b-807a-69f8dc9f64ef" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-769b-807a-69f8dc9f64ef_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f3d79-a0dd-769b-807a-69f8dc9f64ef" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-769b-807a-69f8dc9f64ef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-7b58-a5c2-4951042603c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f3d79-a0dd-769b-807a-69f8dc9f64ef" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-7b58-a5c2-4951042603c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_019f3d79-a0dd-7500-aeee-d3dd9a8a5c4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-7b58-a5c2-4951042603c4" xlink:to="loc_us-gaap_PrivatePlacementMember_019f3d79-a0dd-7500-aeee-d3dd9a8a5c4c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ExchangeAgreementUnitsWithoutWarrantsMember_019f3d79-a0dd-75c5-a31c-196e0b948cd0" xlink:href="toi-20260630.xsd#toi_ExchangeAgreementUnitsWithoutWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-7b58-a5c2-4951042603c4" xlink:to="loc_toi_ExchangeAgreementUnitsWithoutWarrantsMember_019f3d79-a0dd-75c5-a31c-196e0b948cd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019f3d79-a0dd-707b-a7b0-d77be291cd3f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-70b1-8247-d30c63d515b7" xlink:to="loc_srt_CounterpartyNameAxis_019f3d79-a0dd-707b-a7b0-d77be291cd3f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-707b-a7b0-d77be291cd3f_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019f3d79-a0dd-707b-a7b0-d77be291cd3f" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-707b-a7b0-d77be291cd3f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-719e-8dc4-0bf4f7afa456" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019f3d79-a0dd-707b-a7b0-d77be291cd3f" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-719e-8dc4-0bf4f7afa456" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CertainConvertibleDebtLendersMember_019f3d79-a0dd-79a5-a51e-e595c3416522" xlink:href="toi-20260630.xsd#toi_CertainConvertibleDebtLendersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-719e-8dc4-0bf4f7afa456" xlink:to="loc_toi_CertainConvertibleDebtLendersMember_019f3d79-a0dd-79a5-a51e-e595c3416522" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_019f3d79-a0dd-7f71-b70b-989250ec2e46" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-70b1-8247-d30c63d515b7" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_019f3d79-a0dd-7f71-b70b-989250ec2e46" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7f71-b70b-989250ec2e46_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_019f3d79-a0dd-7f71-b70b-989250ec2e46" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7f71-b70b-989250ec2e46_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7c6c-8388-e366b3ab3691" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_019f3d79-a0dd-7f71-b70b-989250ec2e46" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7c6c-8388-e366b3ab3691" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CommonWarrantsMember_019f3d79-a0dd-78b6-bb49-211c2a6efee3" xlink:href="toi-20260630.xsd#toi_CommonWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7c6c-8388-e366b3ab3691" xlink:to="loc_toi_CommonWarrantsMember_019f3d79-a0dd-78b6-bb49-211c2a6efee3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019fd0c5-3723-745f-9050-203971eed18a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-70b1-8247-d30c63d515b7" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019fd0c5-3723-745f-9050-203971eed18a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fd0c5-3723-745f-9050-203971eed18a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fd0c5-3723-745f-9050-203971eed18a" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fd0c5-3723-745f-9050-203971eed18a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fd0c5-3723-719d-92cc-8c60320ec285" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fd0c5-3723-745f-9050-203971eed18a" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fd0c5-3723-719d-92cc-8c60320ec285" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019fd0c5-3723-7efb-b0ad-b44000ebfc4b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019fd0c5-3723-719d-92cc-8c60320ec285" xlink:to="loc_us-gaap_SubsequentEventMember_019fd0c5-3723-7efb-b0ad-b44000ebfc4b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f3d79-a0dd-7b55-9bec-b8cb76406c03" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-70b1-8247-d30c63d515b7" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f3d79-a0dd-7b55-9bec-b8cb76406c03" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7b55-9bec-b8cb76406c03_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f3d79-a0dd-7b55-9bec-b8cb76406c03" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7b55-9bec-b8cb76406c03_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7e4b-b5a9-da9d70af54aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019f3d79-a0dd-7b55-9bec-b8cb76406c03" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7e4b-b5a9-da9d70af54aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019f3d79-a0dd-79b7-ac30-0b7ea29b823a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019f3d79-a0dd-7e4b-b5a9-da9d70af54aa" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019f3d79-a0dd-79b7-ac30-0b7ea29b823a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019f3d79-a0dd-7301-b16e-005135defa87" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019f3d79-a0dd-7301-b16e-005135defa87" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019f3d79-a0dd-7cf4-9ee2-c3e7baad5473" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019f3d79-a0dd-7cf4-9ee2-c3e7baad5473" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtInstrumentCovenantMinimumNetQuarterlyRevenues_019f3d79-a0dd-7185-9fec-04dc7ff9a7a0" xlink:href="toi-20260630.xsd#toi_DebtInstrumentCovenantMinimumNetQuarterlyRevenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_toi_DebtInstrumentCovenantMinimumNetQuarterlyRevenues_019f3d79-a0dd-7185-9fec-04dc7ff9a7a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtInstrumentCovenantMinimumCashThresholdRemoval_019f3d79-a0dd-7ea0-8e37-c2cba5455fdd" xlink:href="toi-20260630.xsd#toi_DebtInstrumentCovenantMinimumCashThresholdRemoval"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_toi_DebtInstrumentCovenantMinimumCashThresholdRemoval_019f3d79-a0dd-7ea0-8e37-c2cba5455fdd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_NumberOfEquityOfferings_019f3d79-a0dd-710b-9b96-dc15b1269acc" xlink:href="toi-20260630.xsd#toi_NumberOfEquityOfferings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_toi_NumberOfEquityOfferings_019f3d79-a0dd-710b-9b96-dc15b1269acc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebt_019f3d79-a0dd-798d-8c11-e0fe2af2566d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebt_019f3d79-a0dd-798d-8c11-e0fe2af2566d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019f3d79-a0dd-774a-8c8a-1a08a5c844e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019f3d79-a0dd-774a-8c8a-1a08a5c844e3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtConversionConvertedInstrumentAmount1_019f3d79-a0dd-7d6b-b690-99519c7bc787" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtConversionConvertedInstrumentAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_DebtConversionConvertedInstrumentAmount1_019f3d79-a0dd-7d6b-b690-99519c7bc787" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CommonStockConvertibleSharesIssuable_019f3d79-a0dd-78a9-a8ca-72fba89f3954" xlink:href="toi-20260630.xsd#toi_CommonStockConvertibleSharesIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_toi_CommonStockConvertibleSharesIssuable_019f3d79-a0dd-78a9-a8ca-72fba89f3954" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_019f3d79-a0dd-7f62-9f99-94b15a01b93f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_019f3d79-a0dd-7f62-9f99-94b15a01b93f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfDebt_019fd0c4-a3a2-7a8e-b7c1-dfd102e6631c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_RepaymentsOfDebt_019fd0c4-a3a2-7a8e-b7c1-dfd102e6631c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019f3d79-a0dd-7ca9-934b-add8fffda6ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019f3d79-a0dd-7ca9-934b-add8fffda6ca" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtInstrumentConvertibleConversionCapShares_019f3d79-a0dd-7597-9f3a-0e1df41dcdc3" xlink:href="toi-20260630.xsd#toi_DebtInstrumentConvertibleConversionCapShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_toi_DebtInstrumentConvertibleConversionCapShares_019f3d79-a0dd-7597-9f3a-0e1df41dcdc3" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConversionOfStockSharesIssued1_019f3d79-a0dd-724c-a414-360e3b1d47d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConversionOfStockSharesIssued1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_ConversionOfStockSharesIssued1_019f3d79-a0dd-724c-a414-360e3b1d47d9" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_019f3d79-a0dd-7734-b5ce-53f419b92c49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_019f3d79-a0dd-7734-b5ce-53f419b92c49" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSecuredLinesOfCredit_019f3d79-a0dd-7032-83f9-ebbe9a2f6370" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromSecuredLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_ProceedsFromSecuredLinesOfCredit_019f3d79-a0dd-7032-83f9-ebbe9a2f6370" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019f3d79-a0dd-79de-a024-fd61389145a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_LongTermDebt_019f3d79-a0dd-79de-a024-fd61389145a3" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtInstrumentConvertibleDiscountAmortizationPeriod_019f3d79-a0dd-715d-9b40-3df59552f3ca" xlink:href="toi-20260630.xsd#toi_DebtInstrumentConvertibleDiscountAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_toi_DebtInstrumentConvertibleDiscountAmortizationPeriod_019f3d79-a0dd-715d-9b40-3df59552f3ca" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtInstrumentDebtDiscountRemainingDiscountAmortizationPeriod_019f3d79-a0dd-7170-8955-f698d53ab245" xlink:href="toi-20260630.xsd#toi_DebtInstrumentDebtDiscountRemainingDiscountAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_toi_DebtInstrumentDebtDiscountRemainingDiscountAmortizationPeriod_019f3d79-a0dd-7170-8955-f698d53ab245" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019f3d79-a0dd-73a7-a616-ef1a1cbd28e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019f3d79-a0dd-73a7-a616-ef1a1cbd28e1" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtInstrumentDebtIssuanceCostAmortizationPeriod_019f3d79-a0dd-767c-8a41-e0fb304b561f" xlink:href="toi-20260630.xsd#toi_DebtInstrumentDebtIssuanceCostAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_toi_DebtInstrumentDebtIssuanceCostAmortizationPeriod_019f3d79-a0dd-767c-8a41-e0fb304b561f" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFairValue_019f3d79-a0dd-74ff-9dad-d8fea2956ba6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_DebtInstrumentFairValue_019f3d79-a0dd-74ff-9dad-d8fea2956ba6" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_019f3d79-a0dd-7a96-b7fa-11ca2a0db704" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_019f3d79-a0dd-7a96-b7fa-11ca2a0db704" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_019f3d79-a0dd-796f-84e7-e300f650f0c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_019f3d79-a0dd-796f-84e7-e300f650f0c1" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentIncreaseAccruedInterest_019f3d79-a0dd-7627-8747-c3db6bca2cb7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentIncreaseAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_us-gaap_DebtInstrumentIncreaseAccruedInterest_019f3d79-a0dd-7627-8747-c3db6bca2cb7" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtInstrumentAccruedInterest_019f3d79-a0dd-7b1b-b091-147fca93ed43" xlink:href="toi-20260630.xsd#toi_DebtInstrumentAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7a9e-85d3-de4c062e9378" xlink:to="loc_toi_DebtInstrumentAccruedInterest_019f3d79-a0dd-7b1b-b091-147fca93ed43" xlink:type="arc" order="25"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/DebtSummaryofSeniorSecuredConvertibleNoteDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#DebtSummaryofSeniorSecuredConvertibleNoteDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/DebtSummaryofSeniorSecuredConvertibleNoteDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7fe3-89db-8e1a4f61f271" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-7271-b83d-c314d3f6c644" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7fe3-89db-8e1a4f61f271" xlink:to="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-7271-b83d-c314d3f6c644" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019f3d79-a0dd-75ac-bafb-660e18afcfe8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-7271-b83d-c314d3f6c644" xlink:to="loc_us-gaap_DebtInstrumentAxis_019f3d79-a0dd-75ac-bafb-660e18afcfe8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-75ac-bafb-660e18afcfe8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f3d79-a0dd-75ac-bafb-660e18afcfe8" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-75ac-bafb-660e18afcfe8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-79d2-9b0e-e29ef7945420" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019f3d79-a0dd-75ac-bafb-660e18afcfe8" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-79d2-9b0e-e29ef7945420" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_FacilityAgreementMember_019f3d79-a0dd-7f6d-a20b-4181dfc773c9" xlink:href="toi-20260630.xsd#toi_FacilityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019f3d79-a0dd-79d2-9b0e-e29ef7945420" xlink:to="loc_toi_FacilityAgreementMember_019f3d79-a0dd-7f6d-a20b-4181dfc773c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019f3d79-a0dd-77fa-bf7d-e87271f75abe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019f3d79-a0dd-7271-b83d-c314d3f6c644" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019f3d79-a0dd-77fa-bf7d-e87271f75abe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-77fa-bf7d-e87271f75abe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f3d79-a0dd-77fa-bf7d-e87271f75abe" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-77fa-bf7d-e87271f75abe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-79ad-84df-bc1cb0ecaf66" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019f3d79-a0dd-77fa-bf7d-e87271f75abe" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-79ad-84df-bc1cb0ecaf66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019f3d79-a0dd-70b3-aea9-78e5b75490d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019f3d79-a0dd-79ad-84df-bc1cb0ecaf66" xlink:to="loc_us-gaap_ConvertibleDebtMember_019f3d79-a0dd-70b3-aea9-78e5b75490d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019f3d79-a0dd-7abe-9004-1e7534c81363" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7fe3-89db-8e1a4f61f271" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019f3d79-a0dd-7abe-9004-1e7534c81363" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019f3d79-a0dd-7bc8-b840-442e9b0fa19d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7fe3-89db-8e1a4f61f271" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019f3d79-a0dd-7bc8-b840-442e9b0fa19d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019f3d79-a0dd-7a48-baf3-5781b4d244ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7fe3-89db-8e1a4f61f271" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019f3d79-a0dd-7a48-baf3-5781b4d244ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019f3d79-a0dd-7cb6-8682-3a39eb84f6c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019f3d79-a0dd-7fe3-89db-8e1a4f61f271" xlink:to="loc_us-gaap_LongTermDebt_019f3d79-a0dd-7cb6-8682-3a39eb84f6c4" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/StockholdersEquityDeficitDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#StockholdersEquityDeficitDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/StockholdersEquityDeficitDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019f3d79-a0dd-7fc6-8f79-b935b871be82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019f3d79-a0dd-7fc6-8f79-b935b871be82" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_019f3d79-a0dd-7adb-a993-a46bb141ee4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019f3d79-a0dd-7fc6-8f79-b935b871be82" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_019f3d79-a0dd-7adb-a993-a46bb141ee4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7adb-a993-a46bb141ee4c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_019f3d79-a0dd-7adb-a993-a46bb141ee4c" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7adb-a993-a46bb141ee4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7118-a290-24c592c3f296" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_019f3d79-a0dd-7adb-a993-a46bb141ee4c" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7118-a290-24c592c3f296" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PublicAndPrivateWarrantsMember_019f3d79-a0dd-7763-8629-a519a761c3d2" xlink:href="toi-20260630.xsd#toi_PublicAndPrivateWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7118-a290-24c592c3f296" xlink:to="loc_toi_PublicAndPrivateWarrantsMember_019f3d79-a0dd-7763-8629-a519a761c3d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PublicWarrantsMember_019f3d79-a0dd-793d-b175-e6673080778e" xlink:href="toi-20260630.xsd#toi_PublicWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7118-a290-24c592c3f296" xlink:to="loc_toi_PublicWarrantsMember_019f3d79-a0dd-793d-b175-e6673080778e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PrivateWarrantsMember_019f3d79-a0dd-7fef-a607-ae6e8bfb2c28" xlink:href="toi-20260630.xsd#toi_PrivateWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7118-a290-24c592c3f296" xlink:to="loc_toi_PrivateWarrantsMember_019f3d79-a0dd-7fef-a607-ae6e8bfb2c28" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PreFundedWarrantsMember_019f3d79-a0dd-772e-bd89-04538738f6cb" xlink:href="toi-20260630.xsd#toi_PreFundedWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7118-a290-24c592c3f296" xlink:to="loc_toi_PreFundedWarrantsMember_019f3d79-a0dd-772e-bd89-04538738f6cb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CommonWarrantsMember_019f3d79-a0dd-7e28-a3b1-7555ae32f8dc" xlink:href="toi-20260630.xsd#toi_CommonWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_019f3d79-a0dd-7118-a290-24c592c3f296" xlink:to="loc_toi_CommonWarrantsMember_019f3d79-a0dd-7e28-a3b1-7555ae32f8dc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f3d79-a0dd-7b21-a99c-82e1480a9ca7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019f3d79-a0dd-7fc6-8f79-b935b871be82" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f3d79-a0dd-7b21-a99c-82e1480a9ca7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-7b21-a99c-82e1480a9ca7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f3d79-a0dd-7b21-a99c-82e1480a9ca7" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-7b21-a99c-82e1480a9ca7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-741a-97c6-281b10af988f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f3d79-a0dd-7b21-a99c-82e1480a9ca7" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-741a-97c6-281b10af988f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ExchangeAgreementUnitsWithoutWarrantsMember_019f3d79-a0dd-71d3-b471-d91837693847" xlink:href="toi-20260630.xsd#toi_ExchangeAgreementUnitsWithoutWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-741a-97c6-281b10af988f" xlink:to="loc_toi_ExchangeAgreementUnitsWithoutWarrantsMember_019f3d79-a0dd-71d3-b471-d91837693847" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_019f3d79-a0dd-7628-b50d-21d8e6d4e383" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-741a-97c6-281b10af988f" xlink:to="loc_us-gaap_PrivatePlacementMember_019f3d79-a0dd-7628-b50d-21d8e6d4e383" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_AtTheMarketOfferingMember_019f3d79-a0dd-7448-92e5-3d354eef944d" xlink:href="toi-20260630.xsd#toi_AtTheMarketOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f3d79-a0dd-741a-97c6-281b10af988f" xlink:to="loc_toi_AtTheMarketOfferingMember_019f3d79-a0dd-7448-92e5-3d354eef944d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019f3d79-a0dd-7bc7-93c6-79d58d00c5b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019f3d79-a0dd-7fc6-8f79-b935b871be82" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019f3d79-a0dd-7bc7-93c6-79d58d00c5b7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f3d79-a0dd-7bc7-93c6-79d58d00c5b7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f3d79-a0dd-7bc7-93c6-79d58d00c5b7" xlink:to="loc_us-gaap_ClassOfStockDomain_019f3d79-a0dd-7bc7-93c6-79d58d00c5b7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f3d79-a0dd-72ab-9129-299eb640b91f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f3d79-a0dd-7bc7-93c6-79d58d00c5b7" xlink:to="loc_us-gaap_ClassOfStockDomain_019f3d79-a0dd-72ab-9129-299eb640b91f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019f3d79-a0dd-7856-a04e-77c67c18ebe8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f3d79-a0dd-72ab-9129-299eb640b91f" xlink:to="loc_us-gaap_CommonClassAMember_019f3d79-a0dd-7856-a04e-77c67c18ebe8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019f3d79-a0dd-729a-b6ba-98f4f2b0b197" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019f3d79-a0dd-7fc6-8f79-b935b871be82" xlink:to="loc_srt_CounterpartyNameAxis_019f3d79-a0dd-729a-b6ba-98f4f2b0b197" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-729a-b6ba-98f4f2b0b197_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019f3d79-a0dd-729a-b6ba-98f4f2b0b197" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-729a-b6ba-98f4f2b0b197_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-7c1c-8bd1-3bfb934f0823" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019f3d79-a0dd-729a-b6ba-98f4f2b0b197" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-7c1c-8bd1-3bfb934f0823" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CertainConvertibleDebtLendersMember_019f3d79-a0dd-74c3-af86-2404052df1c5" xlink:href="toi-20260630.xsd#toi_CertainConvertibleDebtLendersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-7c1c-8bd1-3bfb934f0823" xlink:to="loc_toi_CertainConvertibleDebtLendersMember_019f3d79-a0dd-74c3-af86-2404052df1c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_B.RileySecuritiesMember_019f3d79-a0dd-707f-8e77-fc934fb7216c" xlink:href="toi-20260630.xsd#toi_B.RileySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-7c1c-8bd1-3bfb934f0823" xlink:to="loc_toi_B.RileySecuritiesMember_019f3d79-a0dd-707f-8e77-fc934fb7216c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_BTIGMember_019f3d79-a0dd-7fcc-9e77-dca5dd15765c" xlink:href="toi-20260630.xsd#toi_BTIGMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019f3d79-a0dd-7c1c-8bd1-3bfb934f0823" xlink:to="loc_toi_BTIGMember_019f3d79-a0dd-7fcc-9e77-dca5dd15765c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019f3d79-a0dd-7869-8647-44883cb74f89" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_CommonStockSharesIssued_019f3d79-a0dd-7869-8647-44883cb74f89" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019f3d79-a0dd-7407-bc0a-bf94bd7321d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019f3d79-a0dd-7407-bc0a-bf94bd7321d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CommonStockVotesPerShare_019f3d79-a0dd-7d00-b481-3f66282f2cac" xlink:href="toi-20260630.xsd#toi_CommonStockVotesPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_CommonStockVotesPerShare_019f3d79-a0dd-7d00-b481-3f66282f2cac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_019f3d79-a0dd-7cb3-9d82-00bd5534bba7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_DividendsCommonStock_019f3d79-a0dd-7cb3-9d82-00bd5534bba7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019f3d79-a0dd-7e18-b56d-362b7007ad63" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019f3d79-a0dd-7e18-b56d-362b7007ad63" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019f3d79-a0dd-70c5-b4b6-7618b15ac8c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019f3d79-a0dd-70c5-b4b6-7618b15ac8c3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockLiquidationPreference_019f3d79-a0dd-7c6e-a8ba-6ff661e2c13d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockLiquidationPreference"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_PreferredStockLiquidationPreference_019f3d79-a0dd-7c6e-a8ba-6ff661e2c13d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019f3d79-a0dd-7064-b0c5-c421c72bf5e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_019f3d79-a0dd-7064-b0c5-c421c72bf5e2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_019f3d79-a0dd-7256-9b1c-4ab247834435" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_019f3d79-a0dd-7256-9b1c-4ab247834435" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PreferredStockMaximumPercentOfCommonStockAllowed_019f3d79-a0dd-7b5d-ad8e-f8fd177a5d0d" xlink:href="toi-20260630.xsd#toi_PreferredStockMaximumPercentOfCommonStockAllowed"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_PreferredStockMaximumPercentOfCommonStockAllowed_019f3d79-a0dd-7b5d-ad8e-f8fd177a5d0d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClassOfWarrantOrRightExercisableTermFromClosingOfInitialPublicOffering_019f3d79-a0dd-7c2b-9bee-5d679b681d13" xlink:href="toi-20260630.xsd#toi_ClassOfWarrantOrRightExercisableTermFromClosingOfInitialPublicOffering"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_ClassOfWarrantOrRightExercisableTermFromClosingOfInitialPublicOffering_019f3d79-a0dd-7c2b-9bee-5d679b681d13" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_019f3d79-a0dd-7f62-a85e-84cf4029783d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_019f3d79-a0dd-7f62-a85e-84cf4029783d" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_019f3d79-a0dd-72bf-9072-ddf626086170" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_019f3d79-a0dd-72bf-9072-ddf626086170" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight_019f3d79-a0dd-7160-98fd-8d0f32178349" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight_019f3d79-a0dd-7160-98fd-8d0f32178349" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_019f3d79-a0dd-750b-a443-018f4e8bd15b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_019f3d79-a0dd-750b-a443-018f4e8bd15b" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClassOfWarrantOrRightWarrantRedemptionConditionMaximumCommonSharePrice_019f3d79-a0dd-7d74-9a72-06177f5c267b" xlink:href="toi-20260630.xsd#toi_ClassOfWarrantOrRightWarrantRedemptionConditionMaximumCommonSharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_ClassOfWarrantOrRightWarrantRedemptionConditionMaximumCommonSharePrice_019f3d79-a0dd-7d74-9a72-06177f5c267b" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdTradingDays_019f3d79-a0dd-75dd-8429-49d20c61c888" xlink:href="toi-20260630.xsd#toi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdTradingDays_019f3d79-a0dd-75dd-8429-49d20c61c888" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdConsecutiveTradingDays_019f3d79-a0dd-728d-abe4-327db2e086e6" xlink:href="toi-20260630.xsd#toi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdConsecutiveTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdConsecutiveTradingDays_019f3d79-a0dd-728d-abe4-327db2e086e6" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClassOfWarrantOrRightThresholdNumberOfBusinessDaysBeforeSendingNoticeOfRedemptionToWarrantHolders_019f3d79-a0dd-76d8-a2e2-b5dd33691152" xlink:href="toi-20260630.xsd#toi_ClassOfWarrantOrRightThresholdNumberOfBusinessDaysBeforeSendingNoticeOfRedemptionToWarrantHolders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_ClassOfWarrantOrRightThresholdNumberOfBusinessDaysBeforeSendingNoticeOfRedemptionToWarrantHolders_019f3d79-a0dd-76d8-a2e2-b5dd33691152" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClassOfWarrantOrRightRedemptionPriceOfWarrantsOrRights_019f3d79-a0dd-7815-8c2a-2ee4119f0206" xlink:href="toi-20260630.xsd#toi_ClassOfWarrantOrRightRedemptionPriceOfWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_ClassOfWarrantOrRightRedemptionPriceOfWarrantsOrRights_019f3d79-a0dd-7815-8c2a-2ee4119f0206" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClassOfWarrantOrRightRedemptionPeriodWrittenNotice_019f3d79-a0dd-7310-ad1d-43d599b50608" xlink:href="toi-20260630.xsd#toi_ClassOfWarrantOrRightRedemptionPeriodWrittenNotice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_ClassOfWarrantOrRightRedemptionPeriodWrittenNotice_019f3d79-a0dd-7310-ad1d-43d599b50608" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f3d79-a0dd-7991-8884-7dfce34c0206" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_019f3d79-a0dd-7991-8884-7dfce34c0206" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_SaleOfStockSharesIssuedPerUnit_019f3d79-a0dd-7def-a112-a67b4e80355d" xlink:href="toi-20260630.xsd#toi_SaleOfStockSharesIssuedPerUnit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_SaleOfStockSharesIssuedPerUnit_019f3d79-a0dd-7def-a112-a67b4e80355d" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019f3d79-a0dd-7cc8-a573-e59b955d4a81" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019f3d79-a0dd-7cc8-a573-e59b955d4a81" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_019f3d79-a0dd-75b3-ac84-9edf03b38b99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_019f3d79-a0dd-75b3-ac84-9edf03b38b99" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f3d79-a0dd-7b78-b522-b67f73683c0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f3d79-a0dd-7b78-b522-b67f73683c0b" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f3d79-a0dd-7b81-bfc5-36c2d568fa1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019f3d79-a0dd-7b81-bfc5-36c2d568fa1c" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtConversionConvertedInstrumentAmount1_019f3d79-a0dd-74ec-a280-dfab4e1936e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtConversionConvertedInstrumentAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_DebtConversionConvertedInstrumentAmount1_019f3d79-a0dd-74ec-a280-dfab4e1936e5" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CommonStockConvertibleSharesIssuable_019f3d79-a0dd-77fd-bc13-2b6da5717e5a" xlink:href="toi-20260630.xsd#toi_CommonStockConvertibleSharesIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_CommonStockConvertibleSharesIssuable_019f3d79-a0dd-77fd-bc13-2b6da5717e5a" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClassOfWarrantOrRightOwnershipThresholdPercentage_019f3d79-a0dd-7903-883a-c73a38cbfdb5" xlink:href="toi-20260630.xsd#toi_ClassOfWarrantOrRightOwnershipThresholdPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_toi_ClassOfWarrantOrRightOwnershipThresholdPercentage_019f3d79-a0dd-7903-883a-c73a38cbfdb5" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019f3d79-a0dd-7802-9897-0418d0e76f0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019f3d79-a0dd-7802-9897-0418d0e76f0b" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_019f3d79-a0dd-7409-97bc-1f3916f60520" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019f3d79-a0dd-7fc6-b86a-4c1792441e54" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_019f3d79-a0dd-7409-97bc-1f3916f60520" xlink:type="arc" order="31"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/ShareBasedCompensationAdditionalInformationDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#ShareBasedCompensationAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/ShareBasedCompensationAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f3d79-a0dd-7ac9-8dd3-86ac965bf886" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f3d79-a0dd-7ac9-8dd3-86ac965bf886" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019f3d79-a0dd-7655-9f6f-172848f52d2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f3d79-a0dd-7ac9-8dd3-86ac965bf886" xlink:to="loc_us-gaap_AwardTypeAxis_019f3d79-a0dd-7655-9f6f-172848f52d2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7655-9f6f-172848f52d2d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019f3d79-a0dd-7655-9f6f-172848f52d2d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7655-9f6f-172848f52d2d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7d74-9bc5-cabb26b655c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019f3d79-a0dd-7655-9f6f-172848f52d2d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7d74-9bc5-cabb26b655c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019f3d79-a0dd-7c12-9501-01c5a6798804" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7d74-9bc5-cabb26b655c2" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019f3d79-a0dd-7c12-9501-01c5a6798804" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019f3d79-a0dd-7e6c-a949-e8156fcc72a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7d74-9bc5-cabb26b655c2" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019f3d79-a0dd-7e6c-a949-e8156fcc72a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_RestrictedStockUnitsAndAwardsMember_019f3d79-a0dd-74fb-af9b-206d0cc0ee9b" xlink:href="toi-20260630.xsd#toi_RestrictedStockUnitsAndAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7d74-9bc5-cabb26b655c2" xlink:to="loc_toi_RestrictedStockUnitsAndAwardsMember_019f3d79-a0dd-74fb-af9b-206d0cc0ee9b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_MedicalRestrictedStockUnitsRSUsMember_019f3d79-a0dd-74ec-9745-ec867ea4c21d" xlink:href="toi-20260630.xsd#toi_MedicalRestrictedStockUnitsRSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7d74-9bc5-cabb26b655c2" xlink:to="loc_toi_MedicalRestrictedStockUnitsRSUsMember_019f3d79-a0dd-74ec-9745-ec867ea4c21d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_EmployeeStockPurchasePlanMember_019f3d79-a0dd-77bc-8cb7-f0fd4d0f7454" xlink:href="toi-20260630.xsd#toi_EmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7d74-9bc5-cabb26b655c2" xlink:to="loc_toi_EmployeeStockPurchasePlanMember_019f3d79-a0dd-77bc-8cb7-f0fd4d0f7454" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_019f3d79-a0dd-76df-90d1-bf4936060100" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f3d79-a0dd-7ac9-8dd3-86ac965bf886" xlink:to="loc_us-gaap_VestingAxis_019f3d79-a0dd-76df-90d1-bf4936060100" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019f3d79-a0dd-76df-90d1-bf4936060100_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_019f3d79-a0dd-76df-90d1-bf4936060100" xlink:to="loc_us-gaap_VestingDomain_019f3d79-a0dd-76df-90d1-bf4936060100_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019f3d79-a0dd-7018-89fd-82a1195d84ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_019f3d79-a0dd-76df-90d1-bf4936060100" xlink:to="loc_us-gaap_VestingDomain_019f3d79-a0dd-7018-89fd-82a1195d84ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019f3d79-a0dd-71a6-ab52-d9e10c9e0e35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019f3d79-a0dd-7018-89fd-82a1195d84ad" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019f3d79-a0dd-71a6-ab52-d9e10c9e0e35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019f3d79-a0dd-77fe-93a0-37caa56bc03f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019f3d79-a0dd-7018-89fd-82a1195d84ad" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019f3d79-a0dd-77fe-93a0-37caa56bc03f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019f3d79-a0dd-7ac7-bc0c-edba8848a0fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f3d79-a0dd-7ac9-8dd3-86ac965bf886" xlink:to="loc_us-gaap_PlanNameAxis_019f3d79-a0dd-7ac7-bc0c-edba8848a0fa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019f3d79-a0dd-7ac7-bc0c-edba8848a0fa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019f3d79-a0dd-7ac7-bc0c-edba8848a0fa" xlink:to="loc_us-gaap_PlanNameDomain_019f3d79-a0dd-7ac7-bc0c-edba8848a0fa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019f3d79-a0dd-7820-b793-c0f989015ee4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019f3d79-a0dd-7ac7-bc0c-edba8848a0fa" xlink:to="loc_us-gaap_PlanNameDomain_019f3d79-a0dd-7820-b793-c0f989015ee4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_TwoThousandAndTwentyOnePlanMember_019f3d79-a0dd-72a1-b188-c901ac8e37b7" xlink:href="toi-20260630.xsd#toi_TwoThousandAndTwentyOnePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019f3d79-a0dd-7820-b793-c0f989015ee4" xlink:to="loc_toi_TwoThousandAndTwentyOnePlanMember_019f3d79-a0dd-72a1-b188-c901ac8e37b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019f3d79-a0dd-723a-99e2-1a221b7cebb6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_019f3d79-a0dd-723a-99e2-1a221b7cebb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019f3d79-a0dd-78c6-aa9d-4259377670ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019f3d79-a0dd-78c6-aa9d-4259377670ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019f3d79-a0dd-7ed8-a4ae-71fc3bc1e0a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019f3d79-a0dd-7ed8-a4ae-71fc3bc1e0a5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019f3d79-a0dd-7b1d-ae89-393abf37bcf9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019f3d79-a0dd-7b1d-ae89-393abf37bcf9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019f3d79-a0dd-7520-b5e0-7eee6137a2e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019f3d79-a0dd-7520-b5e0-7eee6137a2e1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueVested_019f3d79-a0dd-7c26-ad43-556d759e4d8d" xlink:href="toi-20260630.xsd#toi_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueVested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_toi_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAggregateIntrinsicValueVested_019f3d79-a0dd-7c26-ad43-556d759e4d8d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019f3d79-a0dd-7b5c-94dc-971fbf4d3f2e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_019f3d79-a0dd-7b5c-94dc-971fbf4d3f2e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019f3d79-a0dd-73e0-a352-cda253b5f095" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019f3d79-a0dd-73e0-a352-cda253b5f095" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromStockOptionsExercised_019f3d79-a0dd-7b1d-a08f-b9c74a3f8cae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_ProceedsFromStockOptionsExercised_019f3d79-a0dd-7b1d-a08f-b9c74a3f8cae" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_019f3d79-a0dd-7c6b-a154-cc565c8c6b68" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_019f3d79-a0dd-7c6b-a154-cc565c8c6b68" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_019f3d79-a0dd-7c29-a140-8ee7fc7160ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_019f3d79-a0dd-7c29-a140-8ee7fc7160ba" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019f3d79-a0dd-7209-821f-05dd53b02eb2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019f3d79-a0dd-7209-821f-05dd53b02eb2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019f3d79-a0dd-7e64-a60b-40860b1da444" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019f3d79-a0dd-7e64-a60b-40860b1da444" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019f3d79-a0dd-7470-b869-2607b7081f82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_019f3d79-a0dd-7470-b869-2607b7081f82" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_019f3d79-a0dd-748c-82fe-fbebbd124860" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_019f3d79-a0dd-748c-82fe-fbebbd124860" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019f3d79-a0dd-7676-9fa8-9c7f24bb83f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019f3d79-a0dd-7676-9fa8-9c7f24bb83f7" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationFairValueOfTheLiabilityClassifiedMedicalRSUOutstanding_019f3d79-a0dd-7aef-9e2f-c3ffdc495c00" xlink:href="toi-20260630.xsd#toi_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationFairValueOfTheLiabilityClassifiedMedicalRSUOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_toi_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationFairValueOfTheLiabilityClassifiedMedicalRSUOutstanding_019f3d79-a0dd-7aef-9e2f-c3ffdc495c00" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019f3d79-a0dd-7af5-bd49-0de1afc952b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019f3d79-a0dd-7af5-bd49-0de1afc952b0" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019f3d79-a0dd-713b-80be-c56dcbee7d9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019f3d79-a0dd-713b-80be-c56dcbee7d9c" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_019f3d79-a0dd-7b8d-b039-0839386fc02e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_019f3d79-a0dd-7b8d-b039-0839386fc02e" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_019f3d79-a0dd-7e79-a2c1-4e51dac75c0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7c30-9021-30f4388f5b26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_019f3d79-a0dd-7e79-a2c1-4e51dac75c0b" xlink:type="arc" order="20"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/ShareBasedCompensationSummaryofRSAsDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#ShareBasedCompensationSummaryofRSAsDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/ShareBasedCompensationSummaryofRSAsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7546-91e0-887d2408fac5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f3d79-a0dd-746a-a617-359961f1e6fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7546-91e0-887d2408fac5" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f3d79-a0dd-746a-a617-359961f1e6fc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019f3d79-a0dd-792a-af92-d437d21761f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019f3d79-a0dd-746a-a617-359961f1e6fc" xlink:to="loc_us-gaap_AwardTypeAxis_019f3d79-a0dd-792a-af92-d437d21761f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-792a-af92-d437d21761f1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019f3d79-a0dd-792a-af92-d437d21761f1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-792a-af92-d437d21761f1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7b2f-b17a-d271c4d588ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019f3d79-a0dd-792a-af92-d437d21761f1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7b2f-b17a-d271c4d588ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_RestrictedStockUnitsAndRestrictedStockAwardsMember_019f3d79-a0dd-78d8-8019-d61e3adb113d" xlink:href="toi-20260630.xsd#toi_RestrictedStockUnitsAndRestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7b2f-b17a-d271c4d588ad" xlink:to="loc_toi_RestrictedStockUnitsAndRestrictedStockAwardsMember_019f3d79-a0dd-78d8-8019-d61e3adb113d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_MedicalRestrictedStockUnitsRSUsMember_019f3d79-a0dd-764d-af48-98289b3b3bd2" xlink:href="toi-20260630.xsd#toi_MedicalRestrictedStockUnitsRSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f3d79-a0dd-7b2f-b17a-d271c4d588ad" xlink:to="loc_toi_MedicalRestrictedStockUnitsRSUsMember_019f3d79-a0dd-764d-af48-98289b3b3bd2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019f3d79-a0dd-7250-bd0d-5e5cff1635a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7546-91e0-887d2408fac5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019f3d79-a0dd-7250-bd0d-5e5cff1635a6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019f3d79-a0dd-7f7f-aee8-20c83547ff1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019f3d79-a0dd-7250-bd0d-5e5cff1635a6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019f3d79-a0dd-7f7f-aee8-20c83547ff1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019f3d79-a0dd-7bab-aa21-097233fbe745" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019f3d79-a0dd-7250-bd0d-5e5cff1635a6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019f3d79-a0dd-7bab-aa21-097233fbe745" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019f3d79-a0dd-7cd9-a011-7c7b82e7dbd3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019f3d79-a0dd-7250-bd0d-5e5cff1635a6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019f3d79-a0dd-7cd9-a011-7c7b82e7dbd3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019f3d79-a0dd-74fb-83a8-620acb422f24" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019f3d79-a0dd-7250-bd0d-5e5cff1635a6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019f3d79-a0dd-74fb-83a8-620acb422f24" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019f3d79-a0dd-7ad5-a78e-6a41bb5e94be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019f3d79-a0dd-77c5-8090-152905bef6f2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019f3d79-a0dd-7546-91e0-887d2408fac5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019f3d79-a0dd-77c5-8090-152905bef6f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019f3d79-a0dd-7502-aec2-6c738b4c3aad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019f3d79-a0dd-77c5-8090-152905bef6f2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019f3d79-a0dd-7502-aec2-6c738b4c3aad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019f3d79-a0dd-7548-b169-0ebb3d75a871" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019f3d79-a0dd-77c5-8090-152905bef6f2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019f3d79-a0dd-7548-b169-0ebb3d75a871" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019f3d79-a0dd-782f-9ad3-d21fdd568264" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019f3d79-a0dd-77c5-8090-152905bef6f2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019f3d79-a0dd-782f-9ad3-d21fdd568264" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019f3d79-a0dd-7635-b8c4-2de3ffaea740" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019f3d79-a0dd-77c5-8090-152905bef6f2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019f3d79-a0dd-7635-b8c4-2de3ffaea740" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019f3d79-a0dd-7f08-a552-6f4eaf25c5f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/VariableInterestEntitiesDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#VariableInterestEntitiesDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/VariableInterestEntitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityLineItems_019f3d79-a0de-76d9-aab2-a6e5365595cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019f3d79-a0de-73ae-9b91-49bd82a0f5c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfVariableInterestEntitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019f3d79-a0de-76d9-aab2-a6e5365595cb" xlink:to="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019f3d79-a0de-73ae-9b91-49bd82a0f5c9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_019f3d79-a0de-7178-a94a-e0dbd30bc232" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019f3d79-a0de-73ae-9b91-49bd82a0f5c9" xlink:to="loc_srt_ConsolidatedEntitiesAxis_019f3d79-a0de-7178-a94a-e0dbd30bc232" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019f3d79-a0de-7178-a94a-e0dbd30bc232_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019f3d79-a0de-7178-a94a-e0dbd30bc232" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019f3d79-a0de-7178-a94a-e0dbd30bc232_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_019f3d79-a0de-7abf-baaf-87ac425d5852" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_019f3d79-a0de-7178-a94a-e0dbd30bc232" xlink:to="loc_srt_ConsolidatedEntitiesDomain_019f3d79-a0de-7abf-baaf-87ac425d5852" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019f3d79-a0de-7e26-8fe2-12ada1d9c5b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableInterestEntityPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_019f3d79-a0de-7abf-baaf-87ac425d5852" xlink:to="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_019f3d79-a0de-7e26-8fe2-12ada1d9c5b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f3d79-a0de-7956-92aa-829d1d812c42" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_019f3d79-a0de-73ae-9b91-49bd82a0f5c9" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f3d79-a0de-7956-92aa-829d1d812c42" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f3d79-a0de-7956-92aa-829d1d812c42_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f3d79-a0de-7956-92aa-829d1d812c42" xlink:to="loc_us-gaap_RelatedPartyDomain_019f3d79-a0de-7956-92aa-829d1d812c42_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019f3d79-a0de-72e2-b8f5-845249f49fa7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019f3d79-a0de-7956-92aa-829d1d812c42" xlink:to="loc_us-gaap_RelatedPartyDomain_019f3d79-a0de-72e2-b8f5-845249f49fa7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019f3d79-a0de-7b04-b452-6d4740c23672" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019f3d79-a0de-72e2-b8f5-845249f49fa7" xlink:to="loc_us-gaap_RelatedPartyMember_019f3d79-a0de-7b04-b452-6d4740c23672" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019f3d79-a0de-7025-a461-99e9658e5ba6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019f3d79-a0de-76d9-aab2-a6e5365595cb" xlink:to="loc_us-gaap_AssetsAbstract_019f3d79-a0de-7025-a461-99e9658e5ba6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_019f3d79-a0de-74ea-a9c7-5e1f6efd66a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f3d79-a0de-7025-a461-99e9658e5ba6" xlink:to="loc_us-gaap_AssetsCurrentAbstract_019f3d79-a0de-74ea-a9c7-5e1f6efd66a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019f3d79-a0de-7c4f-ab79-a3357a15de1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f3d79-a0de-74ea-a9c7-5e1f6efd66a7" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019f3d79-a0de-7c4f-ab79-a3357a15de1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019f3d79-a0de-76a6-aba6-b034e23eab25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f3d79-a0de-74ea-a9c7-5e1f6efd66a7" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019f3d79-a0de-76a6-aba6-b034e23eab25" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherReceivablesNetCurrent_019f3d79-a0de-7997-92c8-d4f4185c9ce9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherReceivablesNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f3d79-a0de-74ea-a9c7-5e1f6efd66a7" xlink:to="loc_us-gaap_OtherReceivablesNetCurrent_019f3d79-a0de-7997-92c8-d4f4185c9ce9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_019f3d79-a0de-7046-b12b-a2e938e979e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f3d79-a0de-74ea-a9c7-5e1f6efd66a7" xlink:to="loc_us-gaap_InventoryNet_019f3d79-a0de-7046-b12b-a2e938e979e5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019f3d79-a0de-7e3d-8af0-64613bb5a117" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f3d79-a0de-74ea-a9c7-5e1f6efd66a7" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019f3d79-a0de-7e3d-8af0-64613bb5a117" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_019f3d79-a0de-71ae-8405-fb4430bf726f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019f3d79-a0de-74ea-a9c7-5e1f6efd66a7" xlink:to="loc_us-gaap_AssetsCurrent_019f3d79-a0de-71ae-8405-fb4430bf726f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019f3d79-a0de-78dd-a9e5-88cd3196e2ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f3d79-a0de-7025-a461-99e9658e5ba6" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019f3d79-a0de-78dd-a9e5-88cd3196e2ac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_019f3d79-a0de-7f89-87bb-259e70951629" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f3d79-a0de-7025-a461-99e9658e5ba6" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_019f3d79-a0de-7f89-87bb-259e70951629" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019f3d79-a0de-7252-a94a-bbe176cc1cd0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f3d79-a0de-7025-a461-99e9658e5ba6" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019f3d79-a0de-7252-a94a-bbe176cc1cd0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019f3d79-a0de-73b3-8934-ad5b96838919" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f3d79-a0de-7025-a461-99e9658e5ba6" xlink:to="loc_us-gaap_Goodwill_019f3d79-a0de-73b3-8934-ad5b96838919" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019f3d79-a0de-7599-987d-c94e58756adb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019f3d79-a0de-7025-a461-99e9658e5ba6" xlink:to="loc_us-gaap_Assets_019f3d79-a0de-7599-987d-c94e58756adb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_019f3d79-a0de-7621-b91d-ae399b15fff8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_019f3d79-a0de-76d9-aab2-a6e5365595cb" xlink:to="loc_us-gaap_LiabilitiesAbstract_019f3d79-a0de-7621-b91d-ae399b15fff8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_019f3d79-a0de-71cc-bbdf-2ed6fbc01fca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f3d79-a0de-7621-b91d-ae399b15fff8" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_019f3d79-a0de-71cc-bbdf-2ed6fbc01fca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_019f3d79-a0de-741d-a51a-3fd2b14799d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f3d79-a0de-71cc-bbdf-2ed6fbc01fca" xlink:to="loc_us-gaap_AccountsPayableCurrent_019f3d79-a0de-741d-a51a-3fd2b14799d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_019f3d79-a0de-731d-944d-d6b1e8ee80a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f3d79-a0de-71cc-bbdf-2ed6fbc01fca" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_019f3d79-a0de-731d-944d-d6b1e8ee80a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesCurrent_019f3d79-a0de-78ab-94d2-6cc9851e0588" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f3d79-a0de-71cc-bbdf-2ed6fbc01fca" xlink:to="loc_us-gaap_OtherLiabilitiesCurrent_019f3d79-a0de-78ab-94d2-6cc9851e0588" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_019f3d79-a0de-7fb8-82f0-6b89f5b2113f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019f3d79-a0de-71cc-bbdf-2ed6fbc01fca" xlink:to="loc_us-gaap_LiabilitiesCurrent_019f3d79-a0de-7fb8-82f0-6b89f5b2113f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_019f3d79-a0de-724b-a762-7700dd8b8e09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f3d79-a0de-7621-b91d-ae399b15fff8" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_019f3d79-a0de-724b-a762-7700dd8b8e09" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_019f3d79-a0de-75cf-baac-d7b158bb4f9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_019f3d79-a0de-7621-b91d-ae399b15fff8" xlink:to="loc_us-gaap_Liabilities_019f3d79-a0de-75cf-baac-d7b158bb4f9f" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/GoodwillandIntangibleAssetsSummaryofIntangibleAssetsNetDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#GoodwillandIntangibleAssetsSummaryofIntangibleAssetsNetDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/GoodwillandIntangibleAssetsSummaryofIntangibleAssetsNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019f3d79-a194-7474-a7f5-15faa5d0498f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019f3d79-a194-7db4-9dfb-6ab7179902ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019f3d79-a194-7474-a7f5-15faa5d0498f" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019f3d79-a194-7db4-9dfb-6ab7179902ce" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f3d79-a194-766b-b98a-77d03c6f1104" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019f3d79-a194-7db4-9dfb-6ab7179902ce" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f3d79-a194-766b-b98a-77d03c6f1104" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f3d79-a194-766b-b98a-77d03c6f1104_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f3d79-a194-766b-b98a-77d03c6f1104" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f3d79-a194-766b-b98a-77d03c6f1104_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f3d79-a194-7020-8138-60d2788544a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019f3d79-a194-766b-b98a-77d03c6f1104" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f3d79-a194-7020-8138-60d2788544a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerContractsMember_019f3d79-a194-7692-a009-ea27ce997355" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f3d79-a194-7020-8138-60d2788544a3" xlink:to="loc_us-gaap_CustomerContractsMember_019f3d79-a194-7692-a009-ea27ce997355" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_019f3d79-a194-74f7-a2f2-f4b8628b5859" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f3d79-a194-7020-8138-60d2788544a3" xlink:to="loc_us-gaap_TradeNamesMember_019f3d79-a194-74f7-a2f2-f4b8628b5859" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019f3d79-a194-7d46-9ed2-278e188fb408" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f3d79-a194-7020-8138-60d2788544a3" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019f3d79-a194-7d46-9ed2-278e188fb408" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClinicalContractsAndNoncompeteAgreementsMember_019f3d79-a194-7d68-bc92-9aa85248033d" xlink:href="toi-20260630.xsd#toi_ClinicalContractsAndNoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019f3d79-a194-7020-8138-60d2788544a3" xlink:to="loc_toi_ClinicalContractsAndNoncompeteAgreementsMember_019f3d79-a194-7d68-bc92-9aa85248033d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_019f3d79-a194-7f2b-9041-0fe60fab9216" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019f3d79-a194-7474-a7f5-15faa5d0498f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1_019f3d79-a194-7f2b-9041-0fe60fab9216" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019f3d79-a194-7adf-b0a1-f07ba35c6f91" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019f3d79-a194-7474-a7f5-15faa5d0498f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019f3d79-a194-7adf-b0a1-f07ba35c6f91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019f3d79-a194-7ecb-9b0d-5dfa65c60d63" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019f3d79-a194-7adf-b0a1-f07ba35c6f91" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019f3d79-a194-7ecb-9b0d-5dfa65c60d63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019f3d79-a194-75f7-a278-688163d56120" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019f3d79-a194-7adf-b0a1-f07ba35c6f91" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019f3d79-a194-75f7-a278-688163d56120" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019f3d79-a194-7683-9adc-bd6ac2bf64c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019f3d79-a194-7adf-b0a1-f07ba35c6f91" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019f3d79-a194-7683-9adc-bd6ac2bf64c9" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/GoodwillandIntangibleAssetsSummaryofGoodwillDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#GoodwillandIntangibleAssetsSummaryofGoodwillDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/GoodwillandIntangibleAssetsSummaryofGoodwillDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_019f3d79-a194-7b3e-a22c-992a1d215833" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_019f3d79-a194-7bad-a8dd-a18e17893261" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_019f3d79-a194-7b3e-a22c-992a1d215833" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_019f3d79-a194-7bad-a8dd-a18e17893261" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitAxis_019f3d79-a194-73e9-bc43-390a81d624fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportingUnitAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_019f3d79-a194-7bad-a8dd-a18e17893261" xlink:to="loc_us-gaap_ReportingUnitAxis_019f3d79-a194-73e9-bc43-390a81d624fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-73e9-bc43-390a81d624fd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReportingUnitAxis_019f3d79-a194-73e9-bc43-390a81d624fd" xlink:to="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-73e9-bc43-390a81d624fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7bc4-b0f8-a840b38b587e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReportingUnitAxis_019f3d79-a194-73e9-bc43-390a81d624fd" xlink:to="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7bc4-b0f8-a840b38b587e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PatientServicesSegmentMember_019f3d79-a194-7fe6-b26f-a77d0628c848" xlink:href="toi-20260630.xsd#toi_PatientServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7bc4-b0f8-a840b38b587e" xlink:to="loc_toi_PatientServicesSegmentMember_019f3d79-a194-7fe6-b26f-a77d0628c848" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_SpecialtyPharmacyMember_019f3d79-a194-7530-83dc-cd948464a0f1" xlink:href="toi-20260630.xsd#toi_SpecialtyPharmacyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7bc4-b0f8-a840b38b587e" xlink:to="loc_toi_SpecialtyPharmacyMember_019f3d79-a194-7530-83dc-cd948464a0f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClinicalTrialsAndOtherSegmentMember_019f3d79-a194-7cce-b4ea-64c556bedccc" xlink:href="toi-20260630.xsd#toi_ClinicalTrialsAndOtherSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7bc4-b0f8-a840b38b587e" xlink:to="loc_toi_ClinicalTrialsAndOtherSegmentMember_019f3d79-a194-7cce-b4ea-64c556bedccc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019f3d79-a194-77f8-b33b-cee6f624444f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_019f3d79-a194-7b3e-a22c-992a1d215833" xlink:to="loc_us-gaap_Goodwill_019f3d79-a194-77f8-b33b-cee6f624444f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/GoodwillandIntangibleAssetsAdditionalInformationDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#GoodwillandIntangibleAssetsAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/GoodwillandIntangibleAssetsAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_019f3d79-a194-7a1f-83bd-da0cbd319e74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_019f3d79-a194-7255-b4c5-668e69ba9b61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_019f3d79-a194-7a1f-83bd-da0cbd319e74" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_019f3d79-a194-7255-b4c5-668e69ba9b61" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitAxis_019f3d79-a194-7f31-8314-135bfca293ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportingUnitAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_019f3d79-a194-7255-b4c5-668e69ba9b61" xlink:to="loc_us-gaap_ReportingUnitAxis_019f3d79-a194-7f31-8314-135bfca293ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7f31-8314-135bfca293ed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReportingUnitAxis_019f3d79-a194-7f31-8314-135bfca293ed" xlink:to="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7f31-8314-135bfca293ed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7721-98d4-b524e3baea70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReportingUnitDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReportingUnitAxis_019f3d79-a194-7f31-8314-135bfca293ed" xlink:to="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7721-98d4-b524e3baea70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PatientServicesSegmentMember_019f3d79-a194-7a38-b458-0a2baa4f7de7" xlink:href="toi-20260630.xsd#toi_PatientServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7721-98d4-b524e3baea70" xlink:to="loc_toi_PatientServicesSegmentMember_019f3d79-a194-7a38-b458-0a2baa4f7de7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClinicalTrialsAndOtherSegmentMember_019f3d79-a194-7c60-938e-7eae848fbdc1" xlink:href="toi-20260630.xsd#toi_ClinicalTrialsAndOtherSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7721-98d4-b524e3baea70" xlink:to="loc_toi_ClinicalTrialsAndOtherSegmentMember_019f3d79-a194-7c60-938e-7eae848fbdc1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_SpecialtyPharmacyMember_019f3d79-a194-72be-9e0a-36694c3df0a4" xlink:href="toi-20260630.xsd#toi_SpecialtyPharmacyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReportingUnitDomain_019f3d79-a194-7721-98d4-b524e3baea70" xlink:to="loc_toi_SpecialtyPharmacyMember_019f3d79-a194-72be-9e0a-36694c3df0a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_019f3d79-a194-7523-848e-786a8f30537f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_019f3d79-a194-7a1f-83bd-da0cbd319e74" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_019f3d79-a194-7523-848e-786a8f30537f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairedAccumulatedImpairmentLoss_019f3d79-a194-765d-b787-28da71b30b65" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillImpairedAccumulatedImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_019f3d79-a194-7a1f-83bd-da0cbd319e74" xlink:to="loc_us-gaap_GoodwillImpairedAccumulatedImpairmentLoss_019f3d79-a194-765d-b787-28da71b30b65" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/NetLossPerShareSummaryofComputationofDilutedNetLossPerShareDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#NetLossPerShareSummaryofComputationofDilutedNetLossPerShareDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/NetLossPerShareSummaryofComputationofDilutedNetLossPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019f3d79-a194-79e9-abd5-19b152aa8ea1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019f3d79-a194-762c-87d7-bd45fec084d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019f3d79-a194-79e9-abd5-19b152aa8ea1" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019f3d79-a194-762c-87d7-bd45fec084d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019f3d79-a194-73c4-ac75-9f4bc782b5e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019f3d79-a194-762c-87d7-bd45fec084d7" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019f3d79-a194-73c4-ac75-9f4bc782b5e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-73c4-ac75-9f4bc782b5e4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019f3d79-a194-73c4-ac75-9f4bc782b5e4" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-73c4-ac75-9f4bc782b5e4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-71bd-a50a-a5036b73c381" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019f3d79-a194-73c4-ac75-9f4bc782b5e4" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-71bd-a50a-a5036b73c381" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_019f3d79-a194-7fc4-83dd-60ef1a067685" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-71bd-a50a-a5036b73c381" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_019f3d79-a194-7fc4-83dd-60ef1a067685" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019f3d79-a194-74a4-8e9c-813f2725d861" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-71bd-a50a-a5036b73c381" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019f3d79-a194-74a4-8e9c-813f2725d861" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019f3d79-a194-7abc-be90-8092bbb4b687" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-71bd-a50a-a5036b73c381" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019f3d79-a194-7abc-be90-8092bbb4b687" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PublicWarrantsMember_019f3d79-a194-7901-92a2-3afacae3bf85" xlink:href="toi-20260630.xsd#toi_PublicWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-71bd-a50a-a5036b73c381" xlink:to="loc_toi_PublicWarrantsMember_019f3d79-a194-7901-92a2-3afacae3bf85" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PrivateWarrantsMember_019f3d79-a194-7fb2-861c-b8570ca71e1c" xlink:href="toi-20260630.xsd#toi_PrivateWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-71bd-a50a-a5036b73c381" xlink:to="loc_toi_PrivateWarrantsMember_019f3d79-a194-7fb2-861c-b8570ca71e1c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CommonWarrantsWithPrivatePlacementMember_019f3d79-a194-7586-8a2d-3b0f5c0fe436" xlink:href="toi-20260630.xsd#toi_CommonWarrantsWithPrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019f3d79-a194-71bd-a50a-a5036b73c381" xlink:to="loc_toi_CommonWarrantsWithPrivatePlacementMember_019f3d79-a194-7586-8a2d-3b0f5c0fe436" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019f3d79-a194-76b1-9ff3-18a913cfb17d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019f3d79-a194-79e9-abd5-19b152aa8ea1" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019f3d79-a194-76b1-9ff3-18a913cfb17d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/SegmentInformationSummaryofFinancialInformationfortheCompanysSegmentsDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#SegmentInformationSummaryofFinancialInformationfortheCompanysSegmentsDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/SegmentInformationSummaryofFinancialInformationfortheCompanysSegmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-79ad-a039-01660c8b10db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-79ad-a039-01660c8b10db" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019f3d79-a194-7897-8d11-b366311e3811" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-79ad-a039-01660c8b10db" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019f3d79-a194-7897-8d11-b366311e3811" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019f3d79-a194-7897-8d11-b366311e3811_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019f3d79-a194-7897-8d11-b366311e3811" xlink:to="loc_us-gaap_SegmentDomain_019f3d79-a194-7897-8d11-b366311e3811_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019f3d79-a194-7d7a-9373-2a24d5f9e622" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019f3d79-a194-7897-8d11-b366311e3811" xlink:to="loc_us-gaap_SegmentDomain_019f3d79-a194-7d7a-9373-2a24d5f9e622" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PatientServicesSegmentMember_019f3d79-a194-7a5b-95dc-70fd6d15e0a8" xlink:href="toi-20260630.xsd#toi_PatientServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f3d79-a194-7d7a-9373-2a24d5f9e622" xlink:to="loc_toi_PatientServicesSegmentMember_019f3d79-a194-7a5b-95dc-70fd6d15e0a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_SpecialtyPharmacySegmentMember_019f3d79-a194-7cf0-9171-553c942b4ddd" xlink:href="toi-20260630.xsd#toi_SpecialtyPharmacySegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f3d79-a194-7d7a-9373-2a24d5f9e622" xlink:to="loc_toi_SpecialtyPharmacySegmentMember_019f3d79-a194-7cf0-9171-553c942b4ddd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClinicalTrialsAndOtherSegmentMember_019f3d79-a194-73db-9f07-42dbc5515a76" xlink:href="toi-20260630.xsd#toi_ClinicalTrialsAndOtherSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f3d79-a194-7d7a-9373-2a24d5f9e622" xlink:to="loc_toi_ClinicalTrialsAndOtherSegmentMember_019f3d79-a194-73db-9f07-42dbc5515a76" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019f3d79-a194-747e-96c4-7597202cedf2" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-79ad-a039-01660c8b10db" xlink:to="loc_srt_ConsolidationItemsAxis_019f3d79-a194-747e-96c4-7597202cedf2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f3d79-a194-747e-96c4-7597202cedf2_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019f3d79-a194-747e-96c4-7597202cedf2" xlink:to="loc_srt_ConsolidationItemsDomain_019f3d79-a194-747e-96c4-7597202cedf2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f3d79-a194-7b05-9a87-c77d1ae9d894" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019f3d79-a194-747e-96c4-7597202cedf2" xlink:to="loc_srt_ConsolidationItemsDomain_019f3d79-a194-7b05-9a87-c77d1ae9d894" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019f3d79-a194-774c-a86e-2a0ca7174baf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019f3d79-a194-7b05-9a87-c77d1ae9d894" xlink:to="loc_us-gaap_OperatingSegmentsMember_019f3d79-a194-774c-a86e-2a0ca7174baf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaterialReconcilingItemsMember_019f3d79-a194-78a3-ad23-080fa7ccc1a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MaterialReconcilingItemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019f3d79-a194-7b05-9a87-c77d1ae9d894" xlink:to="loc_us-gaap_MaterialReconcilingItemsMember_019f3d79-a194-78a3-ad23-080fa7ccc1a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019f3d79-a194-7e4c-847d-10641a1a535b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-79ad-a039-01660c8b10db" xlink:to="loc_srt_ProductOrServiceAxis_019f3d79-a194-7e4c-847d-10641a1a535b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7e4c-847d-10641a1a535b_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019f3d79-a194-7e4c-847d-10641a1a535b" xlink:to="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7e4c-847d-10641a1a535b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7c3a-8f2f-460be19942f8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019f3d79-a194-7e4c-847d-10641a1a535b" xlink:to="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7c3a-8f2f-460be19942f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_IntravenousIVDrugCostsSegmentMember_019f3d79-a194-7e10-8b79-1f9becd0afba" xlink:href="toi-20260630.xsd#toi_IntravenousIVDrugCostsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7c3a-8f2f-460be19942f8" xlink:to="loc_toi_IntravenousIVDrugCostsSegmentMember_019f3d79-a194-7e10-8b79-1f9becd0afba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClinicianSalariesAndBenefitsSegmentMember_019f3d79-a194-7b34-be77-489fd5edb87e" xlink:href="toi-20260630.xsd#toi_ClinicianSalariesAndBenefitsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7c3a-8f2f-460be19942f8" xlink:to="loc_toi_ClinicianSalariesAndBenefitsSegmentMember_019f3d79-a194-7b34-be77-489fd5edb87e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_NetworkMedicalExpensesMember_019fccd4-5eed-7f07-ad7f-040694ca771f" xlink:href="toi-20260630.xsd#toi_NetworkMedicalExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7c3a-8f2f-460be19942f8" xlink:to="loc_toi_NetworkMedicalExpensesMember_019fccd4-5eed-7f07-ad7f-040694ca771f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_MedicalSuppliesAndOtherSegmentMember_019f3d79-a194-7561-b647-8336fd658be5" xlink:href="toi-20260630.xsd#toi_MedicalSuppliesAndOtherSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7c3a-8f2f-460be19942f8" xlink:to="loc_toi_MedicalSuppliesAndOtherSegmentMember_019f3d79-a194-7561-b647-8336fd658be5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_NetworkMedicalExpensesMember_019fccd4-1e12-7f9c-866c-b96da3149d56" xlink:href="toi-20260630.xsd#toi_NetworkMedicalExpensesMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019f3d79-a194-7bc5-adc6-5890d52f3d38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019f3d79-a194-7bc5-adc6-5890d52f3d38" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_019f3d79-a194-7c9c-beda-8bc4cdaa9d9b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization_019f3d79-a194-7c9c-beda-8bc4cdaa9d9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_019f3d79-a194-7506-a3e5-ecdb3940e413" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_Depreciation_019f3d79-a194-7506-a3e5-ecdb3940e413" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_019f3d79-a194-7a78-9573-1a855c4c7883" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_019f3d79-a194-7a78-9573-1a855c4c7883" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherOperatingIncome_019f3d79-a194-7ad5-ae5d-32140abea961" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherOperatingIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_OtherOperatingIncome_019f3d79-a194-7ad5-ae5d-32140abea961" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019f3d79-a194-7d7c-82a7-00fc01646f5f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019f3d79-a194-7d7c-82a7-00fc01646f5f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019f3d79-a194-755f-8651-58d38fb945f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_DepreciationAndAmortization_019f3d79-a194-755f-8651-58d38fb945f6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019f3d79-a194-7f67-ba76-9ae90e81c094" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_OperatingIncomeLoss_019f3d79-a194-7f67-ba76-9ae90e81c094" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_019f3d79-a194-7f2a-9503-d4c803b38ef9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_InterestExpenseNonoperating_019f3d79-a194-7f2a-9503-d4c803b38ef9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAdjustmentOfWarrants_019f3d79-a194-7b9e-b720-259b9456261e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAdjustmentOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_FairValueAdjustmentOfWarrants_019f3d79-a194-7b9e-b720-259b9456261e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeGainLossOnDerivativeNet_019f3d79-a194-70af-b96c-2579e5ba22f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeGainLossOnDerivativeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_DerivativeGainLossOnDerivativeNet_019f3d79-a194-70af-b96c-2579e5ba22f5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019f3d79-a194-75bb-9384-9dfb9ff7704e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019f3d79-a194-75bb-9384-9dfb9ff7704e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019f3d79-a194-7ca2-8c82-6d402b25f8ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-752d-a144-f7f60da496fb" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019f3d79-a194-7ca2-8c82-6d402b25f8ee" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/SegmentInformationSummaryofAssetsDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#SegmentInformationSummaryofAssetsDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/SegmentInformationSummaryofAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-7e56-b703-fd55c548c5d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-7909-ac3b-a6c1a7e3ae7d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-7e56-b703-fd55c548c5d4" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-7909-ac3b-a6c1a7e3ae7d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019f3d79-a194-72ad-83f0-0642366a9538" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-7909-ac3b-a6c1a7e3ae7d" xlink:to="loc_srt_ConsolidationItemsAxis_019f3d79-a194-72ad-83f0-0642366a9538" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f3d79-a194-72ad-83f0-0642366a9538_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_019f3d79-a194-72ad-83f0-0642366a9538" xlink:to="loc_srt_ConsolidationItemsDomain_019f3d79-a194-72ad-83f0-0642366a9538_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f3d79-a194-7d97-b908-7b728ad6f985" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_019f3d79-a194-72ad-83f0-0642366a9538" xlink:to="loc_srt_ConsolidationItemsDomain_019f3d79-a194-7d97-b908-7b728ad6f985" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019f3d79-a194-78b0-9aed-1797d19de948" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019f3d79-a194-7d97-b908-7b728ad6f985" xlink:to="loc_us-gaap_OperatingSegmentsMember_019f3d79-a194-78b0-9aed-1797d19de948" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaterialReconcilingItemsMember_019f3d79-a194-7263-a889-a93d3e0dc097" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MaterialReconcilingItemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_019f3d79-a194-7d97-b908-7b728ad6f985" xlink:to="loc_us-gaap_MaterialReconcilingItemsMember_019f3d79-a194-7263-a889-a93d3e0dc097" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019f3d79-a194-727f-a7f4-ce1d096e9813" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-7909-ac3b-a6c1a7e3ae7d" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019f3d79-a194-727f-a7f4-ce1d096e9813" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019f3d79-a194-727f-a7f4-ce1d096e9813_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019f3d79-a194-727f-a7f4-ce1d096e9813" xlink:to="loc_us-gaap_SegmentDomain_019f3d79-a194-727f-a7f4-ce1d096e9813_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019f3d79-a194-7114-9874-c353031bbceb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019f3d79-a194-727f-a7f4-ce1d096e9813" xlink:to="loc_us-gaap_SegmentDomain_019f3d79-a194-7114-9874-c353031bbceb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_PatientServicesSegmentMember_019f3d79-a194-7ac6-be5c-d06602908853" xlink:href="toi-20260630.xsd#toi_PatientServicesSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f3d79-a194-7114-9874-c353031bbceb" xlink:to="loc_toi_PatientServicesSegmentMember_019f3d79-a194-7ac6-be5c-d06602908853" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_SpecialtyPharmacySegmentMember_019f3d79-a194-7ab6-9780-7871bea68d69" xlink:href="toi-20260630.xsd#toi_SpecialtyPharmacySegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f3d79-a194-7114-9874-c353031bbceb" xlink:to="loc_toi_SpecialtyPharmacySegmentMember_019f3d79-a194-7ab6-9780-7871bea68d69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_ClinicalTrialsAndOtherSegmentMember_019f3d79-a194-7f19-9b79-9a7067f8f955" xlink:href="toi-20260630.xsd#toi_ClinicalTrialsAndOtherSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019f3d79-a194-7114-9874-c353031bbceb" xlink:to="loc_toi_ClinicalTrialsAndOtherSegmentMember_019f3d79-a194-7f19-9b79-9a7067f8f955" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_019f3d79-a194-7d0a-bc6a-0e8d39b85626" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsAxis"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f3d79-a194-7d0a-bc6a-0e8d39b85626_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_019f3d79-a194-71c2-a027-ae9c45af9b4a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ConsolidationItemsDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_019f3d79-a194-7281-9168-ed1b55e2019a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaterialReconcilingItemsMember_019f3d79-a194-7bf5-a5f1-48c4e32c32d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MaterialReconcilingItemsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019f3d79-a194-7a27-b781-40573ed073e5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019f3d79-a194-7909-ac3b-a6c1a7e3ae7d" xlink:to="loc_srt_ProductOrServiceAxis_019f3d79-a194-7a27-b781-40573ed073e5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7a27-b781-40573ed073e5_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019f3d79-a194-7a27-b781-40573ed073e5" xlink:to="loc_srt_ProductsAndServicesDomain_019f3d79-a194-7a27-b781-40573ed073e5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019f3d79-a194-718a-9849-54b3c47ffc18" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019f3d79-a194-7a27-b781-40573ed073e5" xlink:to="loc_srt_ProductsAndServicesDomain_019f3d79-a194-718a-9849-54b3c47ffc18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_CapitatedAccountsReceivableSegmentMember_019f3d79-a194-7a6a-982d-1531430a7f84" xlink:href="toi-20260630.xsd#toi_CapitatedAccountsReceivableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-718a-9849-54b3c47ffc18" xlink:to="loc_toi_CapitatedAccountsReceivableSegmentMember_019f3d79-a194-7a6a-982d-1531430a7f84" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_FFSAccountsReceivableSegmentMember_019f3d79-a194-7ff2-af6d-e06b69304d6f" xlink:href="toi-20260630.xsd#toi_FFSAccountsReceivableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-718a-9849-54b3c47ffc18" xlink:to="loc_toi_FFSAccountsReceivableSegmentMember_019f3d79-a194-7ff2-af6d-e06b69304d6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_IVDrugInventorySegmentMember_019f3d79-a194-7a93-ae8f-77516f903870" xlink:href="toi-20260630.xsd#toi_IVDrugInventorySegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-718a-9849-54b3c47ffc18" xlink:to="loc_toi_IVDrugInventorySegmentMember_019f3d79-a194-7a93-ae8f-77516f903870" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_OtherAssetsSegmentMember_019f3d79-a194-7790-9a07-fb70827ee9e6" xlink:href="toi-20260630.xsd#toi_OtherAssetsSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-718a-9849-54b3c47ffc18" xlink:to="loc_toi_OtherAssetsSegmentMember_019f3d79-a194-7790-9a07-fb70827ee9e6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_OralDrugAccountsReceivableSegmentMember_019f3d79-a194-711c-b1e8-076372b61eca" xlink:href="toi-20260630.xsd#toi_OralDrugAccountsReceivableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-718a-9849-54b3c47ffc18" xlink:to="loc_toi_OralDrugAccountsReceivableSegmentMember_019f3d79-a194-711c-b1e8-076372b61eca" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_OralDrugInventorySegmentMember_019f3d79-a194-7bb2-be94-b284bd0b2efd" xlink:href="toi-20260630.xsd#toi_OralDrugInventorySegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019f3d79-a194-718a-9849-54b3c47ffc18" xlink:to="loc_toi_OralDrugInventorySegmentMember_019f3d79-a194-7bb2-be94-b284bd0b2efd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019f3d79-a194-7ea3-bcc1-9a655da20b93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019f3d79-a194-7e56-b703-fd55c548c5d4" xlink:to="loc_us-gaap_Assets_019f3d79-a194-7ea3-bcc1-9a655da20b93" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://theoncologyinstitute.com/role/SubsequenteventsDetails" xlink:type="simple" xlink:href="toi-20260630.xsd#SubsequenteventsDetails"/>
  <link:definitionLink xlink:role="http://theoncologyinstitute.com/role/SubsequenteventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019fa385-6a33-7237-844a-624eb8f64b4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:to="loc_us-gaap_SubsequentEventTable_019fa385-6a33-7237-844a-624eb8f64b4d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019fa386-5bf1-7e32-b6b4-6e139b5ca3fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fa385-6a33-7237-844a-624eb8f64b4d" xlink:to="loc_us-gaap_CreditFacilityAxis_019fa386-5bf1-7e32-b6b4-6e139b5ca3fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fa386-5bf1-7e32-b6b4-6e139b5ca3fa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019fa386-5bf1-7e32-b6b4-6e139b5ca3fa" xlink:to="loc_us-gaap_CreditFacilityDomain_019fa386-5bf1-7e32-b6b4-6e139b5ca3fa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019fa386-5bf1-7e4e-8357-3216241667fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019fa386-5bf1-7e32-b6b4-6e139b5ca3fa" xlink:to="loc_us-gaap_CreditFacilityDomain_019fa386-5bf1-7e4e-8357-3216241667fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019fa386-5bf1-7718-a3e0-338a735b98a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019fa386-5bf1-7e4e-8357-3216241667fc" xlink:to="loc_us-gaap_SecuredDebtMember_019fa386-5bf1-7718-a3e0-338a735b98a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019fa386-5bf1-753d-ae69-60effede1c54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fa385-6a33-7237-844a-624eb8f64b4d" xlink:to="loc_us-gaap_DebtInstrumentAxis_019fa386-5bf1-753d-ae69-60effede1c54" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fa386-5bf1-753d-ae69-60effede1c54_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019fa386-5bf1-753d-ae69-60effede1c54" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fa386-5bf1-753d-ae69-60effede1c54_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019fa386-5bf1-7ee1-8853-1d39280e7bd9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019fa386-5bf1-753d-ae69-60effede1c54" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019fa386-5bf1-7ee1-8853-1d39280e7bd9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_TermLoanAgreementMember_019fa386-5bf1-7a94-8356-3a8a2075a022" xlink:href="toi-20260630.xsd#toi_TermLoanAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fa386-5bf1-7ee1-8853-1d39280e7bd9" xlink:to="loc_toi_TermLoanAgreementMember_019fa386-5bf1-7a94-8356-3a8a2075a022" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_FacilityAgreementMember_019fb47a-1eb2-7998-867a-8f29411d564a" xlink:href="toi-20260630.xsd#toi_FacilityAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019fa386-5bf1-7ee1-8853-1d39280e7bd9" xlink:to="loc_toi_FacilityAgreementMember_019fb47a-1eb2-7998-867a-8f29411d564a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019fa386-5bf1-7d86-a1a3-d7d274726cc3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fa385-6a33-7237-844a-624eb8f64b4d" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019fa386-5bf1-7d86-a1a3-d7d274726cc3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fa386-5bf1-7d86-a1a3-d7d274726cc3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fa386-5bf1-7d86-a1a3-d7d274726cc3" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fa386-5bf1-7d86-a1a3-d7d274726cc3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019fa386-5bf1-7c3f-a14e-c2d84acf5eca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019fa386-5bf1-7d86-a1a3-d7d274726cc3" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019fa386-5bf1-7c3f-a14e-c2d84acf5eca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019fa386-5bf1-76cc-8dd4-ef578bf9192b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019fa386-5bf1-7c3f-a14e-c2d84acf5eca" xlink:to="loc_us-gaap_SubsequentEventMember_019fa386-5bf1-76cc-8dd4-ef578bf9192b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019fa386-86e2-798c-aec0-fa864cc778dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fa385-6a33-7237-844a-624eb8f64b4d" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019fa386-86e2-798c-aec0-fa864cc778dc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fa386-86e2-798c-aec0-fa864cc778dc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019fa386-86e2-798c-aec0-fa864cc778dc" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fa386-86e2-798c-aec0-fa864cc778dc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019fa386-86e2-79ad-9b79-73872051cd66" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019fa386-86e2-798c-aec0-fa864cc778dc" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019fa386-86e2-79ad-9b79-73872051cd66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019fa4b8-9af5-7c59-8656-2a38b29e887b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019fa386-86e2-79ad-9b79-73872051cd66" xlink:to="loc_us-gaap_LineOfCreditMember_019fa4b8-9af5-7c59-8656-2a38b29e887b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019fb47a-1eb2-70c5-b077-b03c58dee485" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019fa386-86e2-79ad-9b79-73872051cd66" xlink:to="loc_us-gaap_ConvertibleDebtMember_019fb47a-1eb2-70c5-b077-b03c58dee485" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_VariableRateComponentAxis_019fa4c1-6a60-7809-b95d-9db79f0ebe1c" xlink:href="toi-20260630.xsd#toi_VariableRateComponentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019fa385-6a33-7237-844a-624eb8f64b4d" xlink:to="loc_toi_VariableRateComponentAxis_019fa4c1-6a60-7809-b95d-9db79f0ebe1c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_VariableRateComponentDomain_019fa4c1-6a60-7809-b95d-9db79f0ebe1c_default" xlink:href="toi-20260630.xsd#toi_VariableRateComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_toi_VariableRateComponentAxis_019fa4c1-6a60-7809-b95d-9db79f0ebe1c" xlink:to="loc_toi_VariableRateComponentDomain_019fa4c1-6a60-7809-b95d-9db79f0ebe1c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_VariableRateComponentDomain_019fa4c1-6a60-7249-a2e0-e3f22377d122" xlink:href="toi-20260630.xsd#toi_VariableRateComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_toi_VariableRateComponentAxis_019fa4c1-6a60-7809-b95d-9db79f0ebe1c" xlink:to="loc_toi_VariableRateComponentDomain_019fa4c1-6a60-7249-a2e0-e3f22377d122" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_VariableRateComponentOneMember_019fa4c1-e19f-7b95-b1e9-328ff5a985f9" xlink:href="toi-20260630.xsd#toi_VariableRateComponentOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_toi_VariableRateComponentDomain_019fa4c1-6a60-7249-a2e0-e3f22377d122" xlink:to="loc_toi_VariableRateComponentOneMember_019fa4c1-e19f-7b95-b1e9-328ff5a985f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_VariableRateComponentTwoMember_019fa4c2-0172-7e0c-9be5-6f1f1d3f93f9" xlink:href="toi-20260630.xsd#toi_VariableRateComponentTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_toi_VariableRateComponentDomain_019fa4c1-6a60-7249-a2e0-e3f22377d122" xlink:to="loc_toi_VariableRateComponentTwoMember_019fa4c2-0172-7e0c-9be5-6f1f1d3f93f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019fa386-5bf1-7ad4-b244-78b7ab6e5afc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019fa386-5bf1-7ad4-b244-78b7ab6e5afc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019fa4be-663a-78a7-af3a-73ec13a4ad92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019fa4be-663a-78a7-af3a-73ec13a4ad92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtInstrumentCovenantMinimumNetRevenue_019fa386-5bf1-7eaa-aa50-bcc9ecea7466" xlink:href="toi-20260630.xsd#toi_DebtInstrumentCovenantMinimumNetRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:to="loc_toi_DebtInstrumentCovenantMinimumNetRevenue_019fa386-5bf1-7eaa-aa50-bcc9ecea7466" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_toi_DebtInstrumentQuarterlyAmortizationRate_019fc57c-305c-7211-b2eb-8a63dfd20931" xlink:href="toi-20260630.xsd#toi_DebtInstrumentQuarterlyAmortizationRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:to="loc_toi_DebtInstrumentQuarterlyAmortizationRate_019fc57c-305c-7211-b2eb-8a63dfd20931" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fa386-5bf1-7bb7-9c88-2063b56ed986" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019fa386-5bf1-7bb7-9c88-2063b56ed986" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019fb47a-1eb2-7682-9d69-d02882162f57" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019fb47a-1eb2-7682-9d69-d02882162f57" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_019fd0db-45d2-7cfb-a5fb-8ee05b952666" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_019fd0db-45d2-7cfb-a5fb-8ee05b952666" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019fd0db-45d2-7116-93ea-c0bbdfb2bed7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019fa385-6a34-79d1-9f6e-97557fb8bd45" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019fd0db-45d2-7116-93ea-c0bbdfb2bed7" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-roles-2026.xsd#eedm1"/>
  <link:definitionLink xlink:role="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherReceivablesNetCurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherReceivablesNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract" xlink:to="loc_us-gaap_OtherReceivablesNetCurrent" xlink:type="arc" order="101"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent" xlink:type="arc" order="41"/>
  </link:definitionLink>
</link:linkbase>
