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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:01a0355c-6d4e-76df-ba90-98852c7d2074,g:ea33285e-bc48-44dd-961f-09b5428f3bd6-->
<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://www.omniab.com/role/CoverPage" xlink:type="simple" xlink:href="oabi-20260930.xsd#CoverPage"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/CoverPage" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_01a0355c-6dec-7260-9a83-71b7ca542efc" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_DocumentInformationTable_01a0355c-6dec-7260-9a83-71b7ca542efc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_01a0355c-6dec-7dd4-ab32-bd6d41245dab" 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_01a0355c-6dec-7260-9a83-71b7ca542efc" xlink:to="loc_us-gaap_StatementClassOfStockAxis_01a0355c-6dec-7dd4-ab32-bd6d41245dab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a0355c-6dec-7dd4-ab32-bd6d41245dab_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_01a0355c-6dec-7dd4-ab32-bd6d41245dab" xlink:to="loc_us-gaap_ClassOfStockDomain_01a0355c-6dec-7dd4-ab32-bd6d41245dab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a0355c-6dec-7e92-9218-047579ab07d3" 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_01a0355c-6dec-7dd4-ab32-bd6d41245dab" xlink:to="loc_us-gaap_ClassOfStockDomain_01a0355c-6dec-7e92-9218-047579ab07d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a0355c-6dec-7ea5-8048-2df93af80d72" 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_01a0355c-6dec-7e92-9218-047579ab07d3" xlink:to="loc_us-gaap_CommonStockMember_01a0355c-6dec-7ea5-8048-2df93af80d72" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_01a0355c-6dec-76ba-aa48-8cd3ce3ee489" 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_01a0355c-6dec-7e92-9218-047579ab07d3" xlink:to="loc_us-gaap_WarrantMember_01a0355c-6dec-76ba-aa48-8cd3ce3ee489" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_01a0355c-6dec-72b7-af9f-fbdc7eb6b808" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_DocumentType_01a0355c-6dec-72b7-af9f-fbdc7eb6b808" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_01a0355c-6dec-76a6-b63e-1116808efc98" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_DocumentQuarterlyReport_01a0355c-6dec-76a6-b63e-1116808efc98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_01a0355c-6dec-7a4e-9248-c04b61e145f3" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_DocumentPeriodEndDate_01a0355c-6dec-7a4e-9248-c04b61e145f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_01a0355c-6dec-77a8-9548-9d3f8c7eb200" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_DocumentTransitionReport_01a0355c-6dec-77a8-9548-9d3f8c7eb200" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_01a0355c-6dec-7838-9bcc-dda18f95c498" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityFileNumber_01a0355c-6dec-7838-9bcc-dda18f95c498" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_01a0355c-6dec-7f93-a314-410a46f064ee" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityRegistrantName_01a0355c-6dec-7f93-a314-410a46f064ee" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_01a0355c-6dec-7d8e-9c3e-ba2d3aca2652" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityIncorporationStateCountryCode_01a0355c-6dec-7d8e-9c3e-ba2d3aca2652" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_01a0355c-6dec-7122-b854-58bf48e8a41a" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityTaxIdentificationNumber_01a0355c-6dec-7122-b854-58bf48e8a41a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_01a0355c-6dec-78e6-8236-6e5294193089" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityAddressAddressLine1_01a0355c-6dec-78e6-8236-6e5294193089" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_01a0355c-6dec-741b-84b7-235f411c7eb9" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityAddressCityOrTown_01a0355c-6dec-741b-84b7-235f411c7eb9" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_01a0355c-6dec-7e97-aec0-acc3eb10aa45" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityAddressStateOrProvince_01a0355c-6dec-7e97-aec0-acc3eb10aa45" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_01a0355c-6dec-7b89-bbe0-2372d436c637" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityAddressPostalZipCode_01a0355c-6dec-7b89-bbe0-2372d436c637" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_01a0355c-6dec-793c-a120-29be44a88792" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_CityAreaCode_01a0355c-6dec-793c-a120-29be44a88792" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_01a0355c-6dec-7bde-85bc-5d1816416fab" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_LocalPhoneNumber_01a0355c-6dec-7bde-85bc-5d1816416fab" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_01a0355c-6dec-7bc4-b7f5-855665bc5105" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_Security12bTitle_01a0355c-6dec-7bc4-b7f5-855665bc5105" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_01a0355c-6dec-7f6f-adc6-2f71a768a15d" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_TradingSymbol_01a0355c-6dec-7f6f-adc6-2f71a768a15d" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_01a0355c-6dec-7961-ad98-c2a7563f18ef" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_SecurityExchangeName_01a0355c-6dec-7961-ad98-c2a7563f18ef" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_01a0355c-6dec-7a60-9dd7-f0058b99e2c2" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityCurrentReportingStatus_01a0355c-6dec-7a60-9dd7-f0058b99e2c2" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_01a0355c-6dec-7196-8604-82512e507259" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityInteractiveDataCurrent_01a0355c-6dec-7196-8604-82512e507259" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_01a0355c-6dec-75d9-a0b5-ff4849d8eabd" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityFilerCategory_01a0355c-6dec-75d9-a0b5-ff4849d8eabd" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_01a0355c-6dec-76c7-beea-5c2719ff3c68" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntitySmallBusiness_01a0355c-6dec-76c7-beea-5c2719ff3c68" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_01a0355c-6dec-7da3-a377-48637394e303" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityEmergingGrowthCompany_01a0355c-6dec-7da3-a377-48637394e303" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityExTransitionPeriod_01a0355c-6dec-7824-9568-0e9ffd0623b1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityExTransitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityExTransitionPeriod_01a0355c-6dec-7824-9568-0e9ffd0623b1" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_01a0355c-6dec-74fa-a339-a3f4fa2eed3a" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityShellCompany_01a0355c-6dec-74fa-a339-a3f4fa2eed3a" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_01a0355c-6dec-7d8c-9e5b-447d4e338dd5" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_01a0355c-6dec-7d8c-9e5b-447d4e338dd5" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_01a0355c-6dec-7c7d-839d-8a4a30a4d0a4" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_CurrentFiscalYearEndDate_01a0355c-6dec-7c7d-839d-8a4a30a4d0a4" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_01a0355c-6dec-73c5-93a5-67465dbb1cb3" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_AmendmentFlag_01a0355c-6dec-73c5-93a5-67465dbb1cb3" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_01a0355c-6dec-7348-971e-9d8dbd1ba417" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_DocumentFiscalYearFocus_01a0355c-6dec-7348-971e-9d8dbd1ba417" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_01a0355c-6dec-750b-b21a-cd3f60595e30" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_DocumentFiscalPeriodFocus_01a0355c-6dec-750b-b21a-cd3f60595e30" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_01a0355c-6dec-7dd5-be88-8131a3c56422" 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_01a0355c-6dec-71dc-a473-e2911e10ad3a" xlink:to="loc_dei_EntityCentralIndexKey_01a0355c-6dec-7dd5-be88-8131a3c56422" xlink:type="arc" order="29"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="simple" xlink:href="oabi-20260930.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_01a0355c-6dec-7526-a257-64e4848a270b" 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_01a0355c-6dec-7af4-88f6-96e4eae3f8f0" 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_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_StatementTable_01a0355c-6dec-7af4-88f6-96e4eae3f8f0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a0355c-6dec-7fe4-a235-7fe8cf676951" 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_01a0355c-6dec-7af4-88f6-96e4eae3f8f0" xlink:to="loc_srt_ProductOrServiceAxis_01a0355c-6dec-7fe4-a235-7fe8cf676951" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a0355c-6dec-7fe4-a235-7fe8cf676951_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_01a0355c-6dec-7fe4-a235-7fe8cf676951" xlink:to="loc_srt_ProductsAndServicesDomain_01a0355c-6dec-7fe4-a235-7fe8cf676951_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a0355c-6dec-751f-8fe6-de7f7c3a9cda" 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_01a0355c-6dec-7fe4-a235-7fe8cf676951" xlink:to="loc_srt_ProductsAndServicesDomain_01a0355c-6dec-751f-8fe6-de7f7c3a9cda" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_LicenseAndMilestoneRevenueMember_01a0355c-6dec-7908-8cdf-a03a0c102550" xlink:href="oabi-20260930.xsd#oabi_LicenseAndMilestoneRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a0355c-6dec-751f-8fe6-de7f7c3a9cda" xlink:to="loc_oabi_LicenseAndMilestoneRevenueMember_01a0355c-6dec-7908-8cdf-a03a0c102550" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_01a0355c-6dec-7a16-8c1e-4aa954555fb4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a0355c-6dec-751f-8fe6-de7f7c3a9cda" xlink:to="loc_us-gaap_ServiceMember_01a0355c-6dec-7a16-8c1e-4aa954555fb4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_XPlorationRevenueMember_01a0355c-6dec-7ee7-a6a2-5b0c98f238c8" xlink:href="oabi-20260930.xsd#oabi_XPlorationRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a0355c-6dec-751f-8fe6-de7f7c3a9cda" xlink:to="loc_oabi_XPlorationRevenueMember_01a0355c-6dec-7ee7-a6a2-5b0c98f238c8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RoyaltyMember_01a0355c-6dec-7b80-9897-438de8459167" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RoyaltyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a0355c-6dec-751f-8fe6-de7f7c3a9cda" xlink:to="loc_us-gaap_RoyaltyMember_01a0355c-6dec-7b80-9897-438de8459167" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesAbstract_01a0355c-6dec-778d-9ff4-decb344353f8" 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_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_RevenuesAbstract_01a0355c-6dec-778d-9ff4-decb344353f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a0355c-6dec-727a-b18a-6d2a701db915" 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_01a0355c-6dec-778d-9ff4-decb344353f8" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a0355c-6dec-727a-b18a-6d2a701db915" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_01a0355c-6dec-7771-8376-73adc226c87d" 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_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_OperatingExpensesAbstract_01a0355c-6dec-7771-8376-73adc226c87d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_01a0355c-6dec-7bea-8ca8-1a075705bb26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a0355c-6dec-7771-8376-73adc226c87d" xlink:to="loc_us-gaap_CostOfRevenue_01a0355c-6dec-7bea-8ca8-1a075705bb26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_01a0355c-6dec-7de6-ae65-1817ad86e8c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a0355c-6dec-7771-8376-73adc226c87d" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_01a0355c-6dec-7de6-ae65-1817ad86e8c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_01a0355c-6dec-7721-83b2-bc9270355c7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a0355c-6dec-7771-8376-73adc226c87d" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_01a0355c-6dec-7721-83b2-bc9270355c7b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_01a0355c-6dec-7c83-86e8-a046d6275175" 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_OperatingExpensesAbstract_01a0355c-6dec-7771-8376-73adc226c87d" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_01a0355c-6dec-7c83-86e8-a046d6275175" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherOperatingIncomeExpenseNet_01a0355c-6dec-7b2d-809d-a7841f55082d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherOperatingIncomeExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a0355c-6dec-7771-8376-73adc226c87d" xlink:to="loc_us-gaap_OtherOperatingIncomeExpenseNet_01a0355c-6dec-7b2d-809d-a7841f55082d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_01a0355c-6dec-7af4-9d95-56ab44fbcc28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a0355c-6dec-7771-8376-73adc226c87d" xlink:to="loc_us-gaap_OperatingExpenses_01a0355c-6dec-7af4-9d95-56ab44fbcc28" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_01a0355c-6dec-73df-a292-1dc206eff339" 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_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_OperatingIncomeLoss_01a0355c-6dec-73df-a292-1dc206eff339" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpenseAbstract_01a0355c-6dec-7e18-9f3e-19c5e65ae691" 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_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_NonoperatingIncomeExpenseAbstract_01a0355c-6dec-7e18-9f3e-19c5e65ae691" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterest_01a0355c-6dec-7ecd-9576-2bd1377ee86a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentIncomeInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_01a0355c-6dec-7e18-9f3e-19c5e65ae691" xlink:to="loc_us-gaap_InvestmentIncomeInterest_01a0355c-6dec-7ecd-9576-2bd1377ee86a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_01a0355c-6dec-753e-bab4-a89d2d8263bd" 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_01a0355c-6dec-7e18-9f3e-19c5e65ae691" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_01a0355c-6dec-753e-bab4-a89d2d8263bd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_01a0355c-6dec-74f1-9e55-04c3f76133ea" 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_01a0355c-6dec-7e18-9f3e-19c5e65ae691" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_01a0355c-6dec-74f1-9e55-04c3f76133ea" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_01a0355c-6dec-792c-9780-dc34e55f687e" 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_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_01a0355c-6dec-792c-9780-dc34e55f687e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_01a0355c-6dec-784f-a778-e33f71cf9800" 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_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_01a0355c-6dec-784f-a778-e33f71cf9800" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_01a0355c-6dec-7f47-8988-10911284c6b7" 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_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_NetIncomeLoss_01a0355c-6dec-7f47-8988-10911284c6b7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_01a0355c-6dec-7abd-95e2-95e89eb593b0" 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_StatementLineItems_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_EarningsPerShareBasic_01a0355c-6dec-7abd-95e2-95e89eb593b0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_01a0355c-6dec-7cea-be29-2dadadfd7ae1" 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_StatementLineItems_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_EarningsPerShareDiluted_01a0355c-6dec-7cea-be29-2dadadfd7ae1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_01a0355c-6dec-76aa-bc0a-b51f9887c338" 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_StatementLineItems_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_01a0355c-6dec-76aa-bc0a-b51f9887c338" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_01a0355c-6dec-7ce1-a9e7-848587e062a9" 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_StatementLineItems_01a0355c-6dec-7526-a257-64e4848a270b" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_01a0355c-6dec-7ce1-a9e7-848587e062a9" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="oabi-20260930.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_01a0355c-6dec-76db-b4dd-e13455246eb7" 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_01a0355c-6dec-7191-a57c-fd646d3038e4" 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_01a0355c-6dec-76db-b4dd-e13455246eb7" xlink:to="loc_us-gaap_StatementTable_01a0355c-6dec-7191-a57c-fd646d3038e4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_01a0355c-6dec-7ec3-a429-c660e5316979" 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_01a0355c-6dec-7191-a57c-fd646d3038e4" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_01a0355c-6dec-7ec3-a429-c660e5316979" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a0355c-6dec-7ec3-a429-c660e5316979_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_01a0355c-6dec-7ec3-a429-c660e5316979" xlink:to="loc_us-gaap_EquityComponentDomain_01a0355c-6dec-7ec3-a429-c660e5316979_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a0355c-6dec-7778-8f6f-d1ba5f6a6a7f" 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_01a0355c-6dec-7ec3-a429-c660e5316979" xlink:to="loc_us-gaap_EquityComponentDomain_01a0355c-6dec-7778-8f6f-d1ba5f6a6a7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a0355c-6dec-716a-b61d-9de07dcb8e94" 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_01a0355c-6dec-7778-8f6f-d1ba5f6a6a7f" xlink:to="loc_us-gaap_CommonStockMember_01a0355c-6dec-716a-b61d-9de07dcb8e94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_01a0355c-6dec-736b-9aa5-5c7b72368c8a" 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_01a0355c-6dec-7778-8f6f-d1ba5f6a6a7f" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_01a0355c-6dec-736b-9aa5-5c7b72368c8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a0355c-6dec-7aba-84e9-2bc9a44d1c20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a0355c-6dec-7778-8f6f-d1ba5f6a6a7f" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a0355c-6dec-7aba-84e9-2bc9a44d1c20" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_01a0355c-6dec-7816-ad0a-56bfcfa52711" 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_01a0355c-6dec-7778-8f6f-d1ba5f6a6a7f" xlink:to="loc_us-gaap_RetainedEarningsMember_01a0355c-6dec-7816-ad0a-56bfcfa52711" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_01a144f2-1e33-7c4e-a907-71522c0e3972" 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_StatementTable_01a0355c-6dec-7191-a57c-fd646d3038e4" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_01a144f2-1e33-7c4e-a907-71522c0e3972" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a144f2-1e33-7c4e-a907-71522c0e3972_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_01a144f2-1e33-7c4e-a907-71522c0e3972" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a144f2-1e33-7c4e-a907-71522c0e3972_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a144f2-1e33-7537-afc9-255adaacfb98" 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_01a144f2-1e33-7c4e-a907-71522c0e3972" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a144f2-1e33-7537-afc9-255adaacfb98" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_01a144f2-1e33-7bd9-97f7-f59dddcd7ffa" 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_01a144f2-1e33-7537-afc9-255adaacfb98" xlink:to="loc_us-gaap_PrivatePlacementMember_01a144f2-1e33-7bd9-97f7-f59dddcd7ffa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0355c-6dec-738c-9795-b0a40d95f801" 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_01a0355c-6dec-76db-b4dd-e13455246eb7" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_01a0355c-6dec-7eba-a1b5-9a78aa6fc4f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_us-gaap_SharesOutstanding_01a0355c-6dec-7eba-a1b5-9a78aa6fc4f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a0355c-6dec-7645-aefd-7f16e215872b" 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_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_us-gaap_StockholdersEquity_01a0355c-6dec-7645-aefd-7f16e215872b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_01a0355c-6dec-7851-87fd-12eb57ac9af8" 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_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_us-gaap_NetIncomeLoss_01a0355c-6dec-7851-87fd-12eb57ac9af8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_01a0355c-6dec-78ab-87bf-8aeb74b4a9a4" 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_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_01a0355c-6dec-78ab-87bf-8aeb74b4a9a4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_01a0355c-6dec-7c7f-a223-a08f8220915b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_01a0355c-6dec-7c7f-a223-a08f8220915b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_01a0355c-6dec-7a58-909d-5cfda4d4abbe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_01a0355c-6dec-7a58-909d-5cfda4d4abbe" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent_01a0355c-6dec-70e0-b463-f3fdd4b9edcf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent_01a0355c-6dec-70e0-b463-f3fdd4b9edcf" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_AdjustmentsToAdditionalPaidInCapitalStockIssuanceCostsReversalOfAccrual_01a0355c-6dec-7bf7-baa4-4abe65d91a06" xlink:href="oabi-20260930.xsd#oabi_AdjustmentsToAdditionalPaidInCapitalStockIssuanceCostsReversalOfAccrual"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_oabi_AdjustmentsToAdditionalPaidInCapitalStockIssuanceCostsReversalOfAccrual_01a0355c-6dec-7bf7-baa4-4abe65d91a06" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_01a144f2-1e33-735e-ac6d-1e08a3612b1e" 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_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_01a144f2-1e33-735e-ac6d-1e08a3612b1e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_01a144f2-1e33-75dc-a029-9e8cc8abd5bd" 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_01a0355c-6dec-738c-9795-b0a40d95f801" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_01a144f2-1e33-75dc-a029-9e8cc8abd5bd" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_01a0355c-6dec-7976-a0cf-e22542d8dbaf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a0355c-6dec-74ff-bc03-eb6ab878a77b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/FairValueMeasurementSummaryofAssetsandLiabilitiesMeasuredatFairValueDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#FairValueMeasurementSummaryofAssetsandLiabilitiesMeasuredatFairValueDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/FairValueMeasurementSummaryofAssetsandLiabilitiesMeasuredatFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0355c-6dec-7bf8-9d43-e446113e8544" 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_01a0355c-6dec-76de-98c0-54631a56df8a" 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_01a0355c-6dec-7bf8-9d43-e446113e8544" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0355c-6dec-76de-98c0-54631a56df8a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a0355c-6dec-7720-8780-a2dd86cdb8cb" 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_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0355c-6dec-76de-98c0-54631a56df8a" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a0355c-6dec-7720-8780-a2dd86cdb8cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0355c-6dec-7720-8780-a2dd86cdb8cb_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_01a0355c-6dec-7720-8780-a2dd86cdb8cb" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0355c-6dec-7720-8780-a2dd86cdb8cb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0355c-6dec-7a1e-b353-8566c3f1970c" 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_01a0355c-6dec-7720-8780-a2dd86cdb8cb" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0355c-6dec-7a1e-b353-8566c3f1970c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_01a0355c-6dec-7f6e-9fa1-9967d7681bb4" 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_01a0355c-6dec-7a1e-b353-8566c3f1970c" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_01a0355c-6dec-7f6e-9fa1-9967d7681bb4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_01a0355c-6dec-7a5f-bf2c-6bb1bd2e0ee3" 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_01a0355c-6dec-7a1e-b353-8566c3f1970c" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_01a0355c-6dec-7a5f-bf2c-6bb1bd2e0ee3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_01a0355c-6dec-7422-985b-32d76134d9ab" 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_01a0355c-6dec-7a1e-b353-8566c3f1970c" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_01a0355c-6dec-7422-985b-32d76134d9ab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_01a0355c-6dec-771e-ad66-6ff600dbc7fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0355c-6dec-76de-98c0-54631a56df8a" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_01a0355c-6dec-771e-ad66-6ff600dbc7fe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0355c-6dec-771e-ad66-6ff600dbc7fe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_01a0355c-6dec-771e-ad66-6ff600dbc7fe" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0355c-6dec-771e-ad66-6ff600dbc7fe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0355c-6dec-7ff7-9e83-195b7bc6edbc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_01a0355c-6dec-771e-ad66-6ff600dbc7fe" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0355c-6dec-7ff7-9e83-195b7bc6edbc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_01a0355c-6dec-71c6-9057-73e07b75a840" 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_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0355c-6dec-7ff7-9e83-195b7bc6edbc" xlink:to="loc_us-gaap_MoneyMarketFundsMember_01a0355c-6dec-71c6-9057-73e07b75a840" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_01a0355c-6dec-7ecb-a7ef-8cba2865a88d" 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_01a0355c-6dec-76de-98c0-54631a56df8a" xlink:to="loc_us-gaap_FinancialInstrumentAxis_01a0355c-6dec-7ecb-a7ef-8cba2865a88d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7ecb-a7ef-8cba2865a88d_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_01a0355c-6dec-7ecb-a7ef-8cba2865a88d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7ecb-a7ef-8cba2865a88d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-73e4-8ae6-d39c138df29f" 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_01a0355c-6dec-7ecb-a7ef-8cba2865a88d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-73e4-8ae6-d39c138df29f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_01a0355c-6dec-7ba1-82fb-141c5425e717" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-73e4-8ae6-d39c138df29f" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_01a0355c-6dec-7ba1-82fb-141c5425e717" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_01a0355c-6dec-7d78-8b1d-c1a1baaa7031" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-73e4-8ae6-d39c138df29f" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_01a0355c-6dec-7d78-8b1d-c1a1baaa7031" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_01a0355c-6dec-7f83-ac86-2d8ff30d144a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-73e4-8ae6-d39c138df29f" xlink:to="loc_us-gaap_CommercialPaperMember_01a0355c-6dec-7f83-ac86-2d8ff30d144a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsAtCarryingValue_01a0355c-6dec-7931-9fa3-f1c9206be3be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0355c-6dec-7bf8-9d43-e446113e8544" xlink:to="loc_us-gaap_CashEquivalentsAtCarryingValue_01a0355c-6dec-7931-9fa3-f1c9206be3be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_01a0355c-6dec-78ef-82f7-7032303f7804" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0355c-6dec-7bf8-9d43-e446113e8544" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_01a0355c-6dec-78ef-82f7-7032303f7804" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityCurrent_01a0355c-6dec-7845-9c3d-a6400968305b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0355c-6dec-7bf8-9d43-e446113e8544" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityCurrent_01a0355c-6dec-7845-9c3d-a6400968305b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_01a0355c-6dec-7925-8e4e-6332a85e60a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0355c-6dec-7bf8-9d43-e446113e8544" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_01a0355c-6dec-7925-8e4e-6332a85e60a8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_01a0355c-6dec-7835-9c6e-08bb0b51209d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0355c-6dec-7bf8-9d43-e446113e8544" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_01a0355c-6dec-7835-9c6e-08bb0b51209d" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/FairValueMeasurementSummaryofLevel3FinancialInstrumentsDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#FairValueMeasurementSummaryofLevel3FinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/FairValueMeasurementSummaryofLevel3FinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_01a0355c-6dec-7ada-99c5-d97034547757" 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_01a0355c-6dec-7231-9486-0ba4d4887adf" 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_01a0355c-6dec-7ada-99c5-d97034547757" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_01a0355c-6dec-7231-9486-0ba4d4887adf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_01a0355c-6dec-71e0-917c-18651ee3746d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_01a0355c-6dec-7231-9486-0ba4d4887adf" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_01a0355c-6dec-71e0-917c-18651ee3746d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0355c-6dec-71e0-917c-18651ee3746d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_01a0355c-6dec-71e0-917c-18651ee3746d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0355c-6dec-71e0-917c-18651ee3746d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0355c-6dec-7cdb-bae7-231118fbccad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_01a0355c-6dec-71e0-917c-18651ee3746d" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0355c-6dec-7cdb-bae7-231118fbccad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_IcagenMember_01a0355c-6dec-7d9b-b103-08c30a17bbe5" xlink:href="oabi-20260930.xsd#oabi_IcagenMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0355c-6dec-7cdb-bae7-231118fbccad" xlink:to="loc_oabi_IcagenMember_01a0355c-6dec-7d9b-b103-08c30a17bbe5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a0355c-6dec-7aad-ab77-cd8a0e059fbd" 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_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_01a0355c-6dec-7231-9486-0ba4d4887adf" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a0355c-6dec-7aad-ab77-cd8a0e059fbd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0355c-6dec-7aad-ab77-cd8a0e059fbd_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_01a0355c-6dec-7aad-ab77-cd8a0e059fbd" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0355c-6dec-7aad-ab77-cd8a0e059fbd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0355c-6dec-730e-95c9-a30204c56961" 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_01a0355c-6dec-7aad-ab77-cd8a0e059fbd" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0355c-6dec-730e-95c9-a30204c56961" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_01a0355c-6dec-7658-87dd-f10c772d3314" 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_01a0355c-6dec-730e-95c9-a30204c56961" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_01a0355c-6dec-7658-87dd-f10c772d3314" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_01a0355c-6dec-7d37-a8b5-051977ea1b89" 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_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_01a0355c-6dec-7231-9486-0ba4d4887adf" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_01a0355c-6dec-7d37-a8b5-051977ea1b89" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_01a0355c-6dec-7d37-a8b5-051977ea1b89_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_01a0355c-6dec-7d37-a8b5-051977ea1b89" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_01a0355c-6dec-7d37-a8b5-051977ea1b89_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_01a0355c-6dec-78fd-87f4-04b6f9968f50" 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_01a0355c-6dec-7d37-a8b5-051977ea1b89" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_01a0355c-6dec-78fd-87f4-04b6f9968f50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_BusinessCombinationContingentConsiderationLiabilityMember_01a0355c-6dec-7248-98dc-d4383ba60b4c" xlink:href="oabi-20260930.xsd#oabi_BusinessCombinationContingentConsiderationLiabilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_01a0355c-6dec-78fd-87f4-04b6f9968f50" xlink:to="loc_oabi_BusinessCombinationContingentConsiderationLiabilityMember_01a0355c-6dec-7248-98dc-d4383ba60b4c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_01a0355c-6dec-7883-9240-1a27ddb30d94" 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_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_01a0355c-6dec-7ada-99c5-d97034547757" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_01a0355c-6dec-7883-9240-1a27ddb30d94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_01a0355c-6dec-71a6-b807-5a33a43cc2ef" 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_01a0355c-6dec-7883-9240-1a27ddb30d94" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_01a0355c-6dec-71a6-b807-5a33a43cc2ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_01a0355c-6dec-7dc8-b192-9bf998dd03de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_01a0355c-6dec-7883-9240-1a27ddb30d94" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_01a0355c-6dec-7dc8-b192-9bf998dd03de" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_01a0355c-6dec-7c23-86ee-a144622692ea" 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_01a0355c-6dec-7883-9240-1a27ddb30d94" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_01a0355c-6dec-7c23-86ee-a144622692ea" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_01a0355c-6dec-77dd-975f-80a0addfdf2b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/ShortTermInvestmentsSummaryofShortTermInvestmentsDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#ShortTermInvestmentsSummaryofShortTermInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/ShortTermInvestmentsSummaryofShortTermInvestmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-7e17-a8c5-eb5ee7ea9529" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_01a0355c-6dec-7f78-aa6a-15b1f0140643" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-7e17-a8c5-eb5ee7ea9529" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_01a0355c-6dec-7f78-aa6a-15b1f0140643" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_01a0355c-6dec-75b7-81a9-7800465a10c3" 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_DebtSecuritiesAvailableForSaleTable_01a0355c-6dec-7f78-aa6a-15b1f0140643" xlink:to="loc_us-gaap_FinancialInstrumentAxis_01a0355c-6dec-75b7-81a9-7800465a10c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-75b7-81a9-7800465a10c3_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_01a0355c-6dec-75b7-81a9-7800465a10c3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-75b7-81a9-7800465a10c3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7f08-8933-0e94cdeeb4e6" 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_01a0355c-6dec-75b7-81a9-7800465a10c3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7f08-8933-0e94cdeeb4e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_01a0355c-6dec-7af6-8e80-35b9bd96a7a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7f08-8933-0e94cdeeb4e6" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_01a0355c-6dec-7af6-8e80-35b9bd96a7a1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_01a0355c-6dec-7ae3-a94c-f8b32d72aa90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7f08-8933-0e94cdeeb4e6" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_01a0355c-6dec-7ae3-a94c-f8b32d72aa90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_01a0355c-6dec-7ee0-8f64-7a143b09dbe9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7f08-8933-0e94cdeeb4e6" xlink:to="loc_us-gaap_CommercialPaperMember_01a0355c-6dec-7ee0-8f64-7a143b09dbe9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_01a0355c-6dec-7923-8874-22aa36e875c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-7e17-a8c5-eb5ee7ea9529" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_01a0355c-6dec-7923-8874-22aa36e875c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_01a0355c-6dec-7d45-9ee7-913124d772d5" 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_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-7e17-a8c5-eb5ee7ea9529" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_01a0355c-6dec-7d45-9ee7-913124d772d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_01a0355c-6dec-7a39-acfb-728e56fd4083" 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_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-7e17-a8c5-eb5ee7ea9529" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_01a0355c-6dec-7a39-acfb-728e56fd4083" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_01a0355c-6dec-7f81-bf18-d89d5057b92d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-7e17-a8c5-eb5ee7ea9529" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_01a0355c-6dec-7f81-bf18-d89d5057b92d" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/ShortTermInvestmentsSummaryofAvailableforSaleInvestmentsinaContinuousLossPositionDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#ShortTermInvestmentsSummaryofAvailableforSaleInvestmentsinaContinuousLossPositionDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/ShortTermInvestmentsSummaryofAvailableforSaleInvestmentsinaContinuousLossPositionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_01a0355c-6dec-7227-8ac5-314e309934dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_01a0355c-6dec-7227-8ac5-314e309934dd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_01a0355c-6dec-7dd3-ad60-ffde9376c327" 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_DebtSecuritiesAvailableForSaleTable_01a0355c-6dec-7227-8ac5-314e309934dd" xlink:to="loc_us-gaap_FinancialInstrumentAxis_01a0355c-6dec-7dd3-ad60-ffde9376c327" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7dd3-ad60-ffde9376c327_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_01a0355c-6dec-7dd3-ad60-ffde9376c327" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7dd3-ad60-ffde9376c327_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7fb2-a2e0-b2e19ca42f6d" 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_01a0355c-6dec-7dd3-ad60-ffde9376c327" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7fb2-a2e0-b2e19ca42f6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_01a14507-46e3-7d1c-9546-4625b61eb63e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7fb2-a2e0-b2e19ca42f6d" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_01a14507-46e3-7d1c-9546-4625b61eb63e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_01a0355c-6dec-74dd-a387-305553d2973f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7fb2-a2e0-b2e19ca42f6d" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_01a0355c-6dec-74dd-a387-305553d2973f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_01a0355c-6dec-7c80-ae27-5e2a1720ceb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a0355c-6dec-7fb2-a2e0-b2e19ca42f6d" xlink:to="loc_us-gaap_CommercialPaperMember_01a0355c-6dec-7c80-ae27-5e2a1720ceb9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions_01a0355c-6dec-73ac-8ccf-9bfa277b212f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions_01a0355c-6dec-73ac-8ccf-9bfa277b212f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_01a0355c-6dec-73c5-9bc2-928a8f407e65" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_01a0355c-6dec-73c5-9bc2-928a8f407e65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_01a0355c-6dec-7e36-a466-f69f23ca3357" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_01a0355c-6dec-7e36-a466-f69f23ca3357" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions_01a0355c-6dec-7edd-8d99-3c66f5127ddb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions_01a0355c-6dec-7edd-8d99-3c66f5127ddb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_01a0355c-6dec-7675-b40e-9834350c3b8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_01a0355c-6dec-7675-b40e-9834350c3b8e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_01a0355c-6dec-7053-be64-f92ccf276162" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_01a0355c-6dec-7053-be64-f92ccf276162" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions_01a0355c-6dec-7811-8d23-10704c2c06d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions_01a0355c-6dec-7811-8d23-10704c2c06d6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_01a0355c-6dec-7c90-8205-2b531e783f12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_01a0355c-6dec-7c90-8205-2b531e783f12" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_01a0355c-6dec-77cc-ace0-98a4fe6ba312" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_01a0355c-6dec-71bc-8d4e-475a32e37395" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_01a0355c-6dec-77cc-ace0-98a4fe6ba312" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/BalanceSheetAccountDetailsAdditionalInformationDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#BalanceSheetAccountDetailsAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/BalanceSheetAccountDetailsAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_01a169ca-fc55-7a5d-8fba-8b1fba1f5ed5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_01a169ca-fc55-7a5d-8fba-8b1fba1f5ed5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a169ca-fc55-7095-99dd-8d845cfa7bb9" 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_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_01a169ca-fc55-7a5d-8fba-8b1fba1f5ed5" xlink:to="loc_srt_ProductOrServiceAxis_01a169ca-fc55-7095-99dd-8d845cfa7bb9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-7095-99dd-8d845cfa7bb9_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_01a169ca-fc55-7095-99dd-8d845cfa7bb9" xlink:to="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-7095-99dd-8d845cfa7bb9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" 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_01a169ca-fc55-7095-99dd-8d845cfa7bb9" xlink:to="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_LicenseRightsResearchServicesAndMaterialRightObligationMember_01a17e46-d22d-7e59-9920-929957d5ec02" xlink:href="oabi-20260930.xsd#oabi_LicenseRightsResearchServicesAndMaterialRightObligationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" xlink:to="loc_oabi_LicenseRightsResearchServicesAndMaterialRightObligationMember_01a17e46-d22d-7e59-9920-929957d5ec02" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ResearchServicesMember_01a169ca-fc55-7bea-beb2-a2089f26f180" xlink:href="oabi-20260930.xsd#oabi_ResearchServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" xlink:to="loc_oabi_ResearchServicesMember_01a169ca-fc55-7bea-beb2-a2089f26f180" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_MaterialRightObligationMember_01a169ca-fc55-7ed4-ae8a-6f124a0317d6" xlink:href="oabi-20260930.xsd#oabi_MaterialRightObligationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" xlink:to="loc_oabi_MaterialRightObligationMember_01a169ca-fc55-7ed4-ae8a-6f124a0317d6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_01a169ca-fc55-7083-b383-69dbfa4d71f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_01a169ca-fc55-7a5d-8fba-8b1fba1f5ed5" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_01a169ca-fc55-7083-b383-69dbfa4d71f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnbilledReceivablesCurrent_01a1453b-14e4-763d-b432-8aa1cc110770" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnbilledReceivablesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_UnbilledReceivablesCurrent_01a1453b-14e4-763d-b432-8aa1cc110770" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_01a1453b-14e4-7b23-a5b6-cf85947e4bab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_01a1453b-14e4-7b23-a5b6-cf85947e4bab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_01a1453b-14e4-7725-a92c-4bdf627b85b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_01a1453b-14e4-7725-a92c-4bdf627b85b6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_01a1453b-14e4-72b3-aade-a2d2a1c12b4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_01a1453b-14e4-72b3-aade-a2d2a1c12b4f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_01a169ca-fc55-7daa-b473-5e370fe8a219" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ContractWithCustomerLiability_01a169ca-fc55-7daa-b473-5e370fe8a219" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_01a169ca-fc55-721f-ac1b-fd52f99a4562" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_01a169ca-fc55-721f-ac1b-fd52f99a4562" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_01a169ca-fc55-7ef8-8520-f3f33238fb84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_01a169ca-fc55-74c8-8810-72a6d608a9cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityNoncurrent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_01a1453b-7b9d-7391-b1f2-30af7e30f270" 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_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_Depreciation_01a1453b-7b9d-7391-b1f2-30af7e30f270" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/BalanceSheetAccountDetailsAdditionalInformationDetails_1" xlink:type="simple" xlink:href="oabi-20260930.xsd#BalanceSheetAccountDetailsAdditionalInformationDetails_1"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/BalanceSheetAccountDetailsAdditionalInformationDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_01a169ca-fc55-7a5d-8fba-8b1fba1f5ed5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_01a169ca-fc55-7a5d-8fba-8b1fba1f5ed5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a169ca-fc55-7095-99dd-8d845cfa7bb9" 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_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_01a169ca-fc55-7a5d-8fba-8b1fba1f5ed5" xlink:to="loc_srt_ProductOrServiceAxis_01a169ca-fc55-7095-99dd-8d845cfa7bb9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-7095-99dd-8d845cfa7bb9_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_01a169ca-fc55-7095-99dd-8d845cfa7bb9" xlink:to="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-7095-99dd-8d845cfa7bb9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" 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_01a169ca-fc55-7095-99dd-8d845cfa7bb9" xlink:to="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_LicenseRightsResearchServicesAndMaterialRightObligationMember_01a17e46-d22d-7e59-9920-929957d5ec02" xlink:href="oabi-20260930.xsd#oabi_LicenseRightsResearchServicesAndMaterialRightObligationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" xlink:to="loc_oabi_LicenseRightsResearchServicesAndMaterialRightObligationMember_01a17e46-d22d-7e59-9920-929957d5ec02" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ResearchServicesMember_01a169ca-fc55-7bea-beb2-a2089f26f180" xlink:href="oabi-20260930.xsd#oabi_ResearchServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" xlink:to="loc_oabi_ResearchServicesMember_01a169ca-fc55-7bea-beb2-a2089f26f180" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_MaterialRightObligationMember_01a169ca-fc55-7ed4-ae8a-6f124a0317d6" xlink:href="oabi-20260930.xsd#oabi_MaterialRightObligationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a169ca-fc55-714c-8c1a-66980d0c8605" xlink:to="loc_oabi_MaterialRightObligationMember_01a169ca-fc55-7ed4-ae8a-6f124a0317d6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnbilledReceivablesCurrent_01a1453b-14e4-763d-b432-8aa1cc110770" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnbilledReceivablesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_UnbilledReceivablesCurrent_01a1453b-14e4-763d-b432-8aa1cc110770" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_01a1453b-14e4-7b23-a5b6-cf85947e4bab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_01a1453b-14e4-7b23-a5b6-cf85947e4bab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_01a1453b-14e4-7725-a92c-4bdf627b85b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_01a1453b-14e4-7725-a92c-4bdf627b85b6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_01a1453b-14e4-72b3-aade-a2d2a1c12b4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_01a1453b-14e4-72b3-aade-a2d2a1c12b4f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_01a169ca-fc55-7daa-b473-5e370fe8a219" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_ContractWithCustomerLiability_01a169ca-fc55-7daa-b473-5e370fe8a219" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_01a169ca-fc55-721f-ac1b-fd52f99a4562" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_01a169ca-fc55-721f-ac1b-fd52f99a4562" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_01a169ca-fc55-7ef8-8520-f3f33238fb84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_01a169ca-fc55-74c8-8810-72a6d608a9cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityNoncurrent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_01a1453b-7b9d-7391-b1f2-30af7e30f270" 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_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_01a169ca-fc55-70a3-b87b-be2046e450f4" xlink:to="loc_us-gaap_Depreciation_01a1453b-7b9d-7391-b1f2-30af7e30f270" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/BalanceSheetAccountDetailsScheduleofPropertyandEquipmentNetDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#BalanceSheetAccountDetailsScheduleofPropertyandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/BalanceSheetAccountDetailsScheduleofPropertyandEquipmentNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0355c-6ded-756e-ab5c-ba73b14415ac" 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_01a0355c-6ded-7fa7-a20a-b7a568ffedfb" 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_01a0355c-6ded-756e-ab5c-ba73b14415ac" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0355c-6ded-7fa7-a20a-b7a568ffedfb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0355c-6ded-72c6-8ba4-5d8bf4ccbc5f" 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_01a0355c-6ded-7fa7-a20a-b7a568ffedfb" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0355c-6ded-72c6-8ba4-5d8bf4ccbc5f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0355c-6ded-72c6-8ba4-5d8bf4ccbc5f_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_01a0355c-6ded-72c6-8ba4-5d8bf4ccbc5f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0355c-6ded-72c6-8ba4-5d8bf4ccbc5f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0355c-6ded-70b6-abca-c62aa831ab3d" 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_01a0355c-6ded-72c6-8ba4-5d8bf4ccbc5f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0355c-6ded-70b6-abca-c62aa831ab3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_01a0355c-6ded-729b-a295-f92a3a68897f" 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_01a0355c-6ded-70b6-abca-c62aa831ab3d" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_01a0355c-6ded-729b-a295-f92a3a68897f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OfficeEquipmentMember_01a0355c-6ded-7035-af32-d680bc2f9356" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OfficeEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0355c-6ded-70b6-abca-c62aa831ab3d" xlink:to="loc_us-gaap_OfficeEquipmentMember_01a0355c-6ded-7035-af32-d680bc2f9356" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_01a0355c-6ded-798f-a2f1-2927443b15a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0355c-6ded-70b6-abca-c62aa831ab3d" xlink:to="loc_us-gaap_ComputerEquipmentMember_01a0355c-6ded-798f-a2f1-2927443b15a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_01a0355c-6ded-72b9-9855-948aafa2e573" 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_01a0355c-6ded-70b6-abca-c62aa831ab3d" xlink:to="loc_us-gaap_ConstructionInProgressMember_01a0355c-6ded-72b9-9855-948aafa2e573" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_01a0355c-6ded-7b77-9124-a3124160c48a" 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_01a0355c-6ded-756e-ab5c-ba73b14415ac" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_01a0355c-6ded-7b77-9124-a3124160c48a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_01a0355c-6ded-7885-91e7-a44a81d4f49b" 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_01a0355c-6ded-756e-ab5c-ba73b14415ac" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_01a0355c-6ded-7885-91e7-a44a81d4f49b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_01a0355c-6ded-7552-8313-53a133e2d595" 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_01a0355c-6ded-756e-ab5c-ba73b14415ac" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_01a0355c-6ded-7552-8313-53a133e2d595" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/GoodwillandIntangibleAssetsNetScheduleofGoodwillandIntangibleAssetsDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#GoodwillandIntangibleAssetsNetScheduleofGoodwillandIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/GoodwillandIntangibleAssetsNetScheduleofGoodwillandIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_01a0355c-6ded-7815-bd1e-6338c2339d83" 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_01a0355c-6ded-7cbb-9e5a-4f7cd9fa9e54" 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_01a0355c-6ded-7815-bd1e-6338c2339d83" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_01a0355c-6ded-7cbb-9e5a-4f7cd9fa9e54" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_01a0355c-6ded-7595-a224-1e5ecd8217d7" 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_ScheduleOfGoodwillTable_01a0355c-6ded-7cbb-9e5a-4f7cd9fa9e54" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_01a0355c-6ded-7595-a224-1e5ecd8217d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a0355c-6ded-7595-a224-1e5ecd8217d7_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_01a0355c-6ded-7595-a224-1e5ecd8217d7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a0355c-6ded-7595-a224-1e5ecd8217d7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a0355c-6ded-76e3-89c6-b1b5aa175b68" 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_01a0355c-6ded-7595-a224-1e5ecd8217d7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a0355c-6ded-76e3-89c6-b1b5aa175b68" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_01a0355c-6ded-7355-8750-f81d3f83b8fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a0355c-6ded-76e3-89c6-b1b5aa175b68" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_01a0355c-6ded-7355-8750-f81d3f83b8fd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_01a0355c-6ded-7ece-9b3b-e886e3f8be4b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_01a0355c-6ded-76e3-89c6-b1b5aa175b68" xlink:to="loc_us-gaap_CustomerRelationshipsMember_01a0355c-6ded-7ece-9b3b-e886e3f8be4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_01a0355c-6ded-7dbf-a963-dc2eee44f7b5" 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_01a0355c-6ded-7815-bd1e-6338c2339d83" xlink:to="loc_us-gaap_Goodwill_01a0355c-6ded-7dbf-a963-dc2eee44f7b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_01a0355c-6ded-7ae2-b345-7f6d7d40a4a2" 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_GoodwillLineItems_01a0355c-6ded-7815-bd1e-6338c2339d83" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_01a0355c-6ded-7ae2-b345-7f6d7d40a4a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_01a0355c-6ded-7f13-b1ca-bf7ead7489d0" 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_01a0355c-6ded-7ae2-b345-7f6d7d40a4a2" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_01a0355c-6ded-7f13-b1ca-bf7ead7489d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_01a0355c-6ded-7bc7-b450-8115097a21a8" 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_01a0355c-6ded-7ae2-b345-7f6d7d40a4a2" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_01a0355c-6ded-7bc7-b450-8115097a21a8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_01a0355c-6ded-77ca-9463-3e176c9592da" 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_FiniteLivedIntangibleAssetsNetAbstract_01a0355c-6ded-7ae2-b345-7f6d7d40a4a2" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_01a0355c-6ded-77ca-9463-3e176c9592da" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetIncludingGoodwill_01a0355c-6ded-715e-98dc-8ab2553432e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IntangibleAssetsNetIncludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_01a0355c-6ded-7815-bd1e-6338c2339d83" xlink:to="loc_us-gaap_IntangibleAssetsNetIncludingGoodwill_01a0355c-6ded-715e-98dc-8ab2553432e0" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/StockholdersEquityDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#StockholdersEquityDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/StockholdersEquityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" 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_01a0355c-6e84-763a-90c5-72dba4bb8091" 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_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0355c-6e84-763a-90c5-72dba4bb8091" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0355c-6e84-75bb-8efc-2325abcd8699" 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_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0355c-6e84-763a-90c5-72dba4bb8091" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0355c-6e84-75bb-8efc-2325abcd8699" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-75bb-8efc-2325abcd8699_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_01a0355c-6e84-75bb-8efc-2325abcd8699" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-75bb-8efc-2325abcd8699_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-7714-b828-a205be7d7c9e" 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_01a0355c-6e84-75bb-8efc-2325abcd8699" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-7714-b828-a205be7d7c9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_PublicWarrantsMember_01a0355c-6e84-77a9-94a5-b3a19395a2fc" xlink:href="oabi-20260930.xsd#oabi_PublicWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-7714-b828-a205be7d7c9e" xlink:to="loc_oabi_PublicWarrantsMember_01a0355c-6e84-77a9-94a5-b3a19395a2fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_EarnoutSharesMember_01a0355c-6e84-7485-bf36-e248e1160c88" xlink:href="oabi-20260930.xsd#oabi_EarnoutSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-7714-b828-a205be7d7c9e" xlink:to="loc_oabi_EarnoutSharesMember_01a0355c-6e84-7485-bf36-e248e1160c88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_SponsorEarnoutSharesMember_01a0355c-6e84-7ace-ae65-68513d26367a" xlink:href="oabi-20260930.xsd#oabi_SponsorEarnoutSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-7714-b828-a205be7d7c9e" xlink:to="loc_oabi_SponsorEarnoutSharesMember_01a0355c-6e84-7ace-ae65-68513d26367a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_PrivatePlacementWarrantsMember_01a0355c-6e84-715b-839c-fd540d548cbd" xlink:href="oabi-20260930.xsd#oabi_PrivatePlacementWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-7714-b828-a205be7d7c9e" xlink:to="loc_oabi_PrivatePlacementWarrantsMember_01a0355c-6e84-715b-839c-fd540d548cbd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ForwardPurchaseWarrantsMember_01a0355c-6e84-74d5-a1d9-5c67a0857c6a" xlink:href="oabi-20260930.xsd#oabi_ForwardPurchaseWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-7714-b828-a205be7d7c9e" xlink:to="loc_oabi_ForwardPurchaseWarrantsMember_01a0355c-6e84-74d5-a1d9-5c67a0857c6a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_BackstopWarrantsMember_01a0355c-6e84-7989-bcbd-caadbd5c1ffa" xlink:href="oabi-20260930.xsd#oabi_BackstopWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e84-7714-b828-a205be7d7c9e" xlink:to="loc_oabi_BackstopWarrantsMember_01a0355c-6e84-7989-bcbd-caadbd5c1ffa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_01a0355c-6e84-7305-a7bc-d11f34b4e5ba" 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_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0355c-6e84-763a-90c5-72dba4bb8091" xlink:to="loc_us-gaap_StatementClassOfStockAxis_01a0355c-6e84-7305-a7bc-d11f34b4e5ba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a0355c-6e84-7305-a7bc-d11f34b4e5ba_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_01a0355c-6e84-7305-a7bc-d11f34b4e5ba" xlink:to="loc_us-gaap_ClassOfStockDomain_01a0355c-6e84-7305-a7bc-d11f34b4e5ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a0355c-6e84-7a09-85ec-76dc7af11bab" 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_01a0355c-6e84-7305-a7bc-d11f34b4e5ba" xlink:to="loc_us-gaap_ClassOfStockDomain_01a0355c-6e84-7a09-85ec-76dc7af11bab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_EarnoutSharesMember_01a0355c-6e84-7998-bd0d-6d0003ae56d8" xlink:href="oabi-20260930.xsd#oabi_EarnoutSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_01a0355c-6e84-7a09-85ec-76dc7af11bab" xlink:to="loc_oabi_EarnoutSharesMember_01a0355c-6e84-7998-bd0d-6d0003ae56d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_01a0355c-6e84-7d5b-b437-455bf197378e" 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_01a0355c-6e84-763a-90c5-72dba4bb8091" xlink:to="loc_us-gaap_PlanNameAxis_01a0355c-6e84-7d5b-b437-455bf197378e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_01a0355c-6e84-7d5b-b437-455bf197378e_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_01a0355c-6e84-7d5b-b437-455bf197378e" xlink:to="loc_us-gaap_PlanNameDomain_01a0355c-6e84-7d5b-b437-455bf197378e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_01a0355c-6e84-7eeb-95b7-cb0941712bdd" 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_01a0355c-6e84-7d5b-b437-455bf197378e" xlink:to="loc_us-gaap_PlanNameDomain_01a0355c-6e84-7eeb-95b7-cb0941712bdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_A2022IncentiveAwardPlanMember_01a0355c-6e84-78e0-8780-c45f0151dc6a" xlink:href="oabi-20260930.xsd#oabi_A2022IncentiveAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_01a0355c-6e84-7eeb-95b7-cb0941712bdd" xlink:to="loc_oabi_A2022IncentiveAwardPlanMember_01a0355c-6e84-78e0-8780-c45f0151dc6a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_A2022LigandServiceProviderAssumedAwardPlanMember_01a0355c-6e84-75ef-89e6-9aaba6ba4e9c" xlink:href="oabi-20260930.xsd#oabi_A2022LigandServiceProviderAssumedAwardPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_01a0355c-6e84-7eeb-95b7-cb0941712bdd" xlink:to="loc_oabi_A2022LigandServiceProviderAssumedAwardPlanMember_01a0355c-6e84-75ef-89e6-9aaba6ba4e9c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_01a0355c-6e84-7d7a-a94f-ea71c337b0ff" 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_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0355c-6e84-763a-90c5-72dba4bb8091" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_01a0355c-6e84-7d7a-a94f-ea71c337b0ff" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a0355c-6e84-7d7a-a94f-ea71c337b0ff_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_01a0355c-6e84-7d7a-a94f-ea71c337b0ff" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a0355c-6e84-7d7a-a94f-ea71c337b0ff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a0355c-6e84-7c14-ab2a-2d13b5e64484" 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_01a0355c-6e84-7d7a-a94f-ea71c337b0ff" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a0355c-6e84-7c14-ab2a-2d13b5e64484" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_AtTheMarketOfferingMember_01a0355c-6e84-70d3-aa1f-a9d7efa8bf92" xlink:href="oabi-20260930.xsd#oabi_AtTheMarketOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a0355c-6e84-7c14-ab2a-2d13b5e64484" xlink:to="loc_oabi_AtTheMarketOfferingMember_01a0355c-6e84-70d3-aa1f-a9d7efa8bf92" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_01a0355c-6e84-7ee1-b557-5f54342a1004" 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_01a0355c-6e84-7c14-ab2a-2d13b5e64484" xlink:to="loc_us-gaap_PrivatePlacementMember_01a0355c-6e84-7ee1-b557-5f54342a1004" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a0355c-6e84-7632-99e9-0340fed5725e" 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_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0355c-6e84-763a-90c5-72dba4bb8091" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_01a0355c-6e84-7632-99e9-0340fed5725e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_01a0355c-6e84-7632-99e9-0340fed5725e_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_01a0355c-6e84-7632-99e9-0340fed5725e" xlink:to="loc_us-gaap_RelatedPartyDomain_01a0355c-6e84-7632-99e9-0340fed5725e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_01a0355c-6e84-754f-a7fe-7361d0ae15cc" 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_01a0355c-6e84-7632-99e9-0340fed5725e" xlink:to="loc_us-gaap_RelatedPartyDomain_01a0355c-6e84-754f-a7fe-7361d0ae15cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_OfficerDirectorEmployeeOrConsultantMember_01a0355c-6e84-7648-9e36-1c9bddb9cc09" xlink:href="oabi-20260930.xsd#oabi_OfficerDirectorEmployeeOrConsultantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_01a0355c-6e84-754f-a7fe-7361d0ae15cc" xlink:to="loc_oabi_OfficerDirectorEmployeeOrConsultantMember_01a0355c-6e84-7648-9e36-1c9bddb9cc09" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_PreferredAndCommonStockShareAuthorized_01a0355c-6e84-7cd0-aac7-df12d9997329" xlink:href="oabi-20260930.xsd#oabi_PreferredAndCommonStockShareAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_PreferredAndCommonStockShareAuthorized_01a0355c-6e84-7cd0-aac7-df12d9997329" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_01a0355c-6e84-70a8-8111-d6fca84e75d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_01a0355c-6e84-70a8-8111-d6fca84e75d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_01a0355c-6e84-7ea9-8e0e-91c6baa562fb" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_01a0355c-6e84-7ea9-8e0e-91c6baa562fb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_01a0355c-6e84-7fec-aee9-8392ed33d9be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_PreferredStockSharesIssued_01a0355c-6e84-7fec-aee9-8392ed33d9be" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_01a0355c-6e84-7d44-8d6f-8e1d6688d39a" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_01a0355c-6e84-7d44-8d6f-8e1d6688d39a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_CommonStockVotingRightsVote_01a0355c-6e84-7543-9e91-8944726b1c9b" xlink:href="oabi-20260930.xsd#oabi_CommonStockVotingRightsVote"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_CommonStockVotingRightsVote_01a0355c-6e84-7543-9e91-8944726b1c9b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdTradingDays_01a0355c-6e84-7c60-b82e-b8de035b7ed2" xlink:href="oabi-20260930.xsd#oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdTradingDays_01a0355c-6e84-7c60-b82e-b8de035b7ed2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdConsecutiveTradingDays_01a0355c-6e84-7842-9d07-64df30342305" xlink:href="oabi-20260930.xsd#oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdConsecutiveTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsThresholdConsecutiveTradingDays_01a0355c-6e84-7842-9d07-64df30342305" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_01a0355c-6e84-7244-b9a0-7bb3052254ff" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_01a0355c-6e84-7244-b9a0-7bb3052254ff" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsSharesVestingUponFirstThresholdPercentage_01a0355c-6e84-7fde-8aaa-621352f5dc35" xlink:href="oabi-20260930.xsd#oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsSharesVestingUponFirstThresholdPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsSharesVestingUponFirstThresholdPercentage_01a0355c-6e84-7fde-8aaa-621352f5dc35" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_VolumeWeightedAverageTradingPrices_01a0355c-6e84-7256-b4b5-41e77abcf74e" xlink:href="oabi-20260930.xsd#oabi_VolumeWeightedAverageTradingPrices"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_VolumeWeightedAverageTradingPrices_01a0355c-6e84-7256-b4b5-41e77abcf74e" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsSharesVestingUponSecondThresholdPercentage_01a0355c-6e84-7289-a6d3-5801db12e913" xlink:href="oabi-20260930.xsd#oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsSharesVestingUponSecondThresholdPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsSharesVestingUponSecondThresholdPercentage_01a0355c-6e84-7289-a6d3-5801db12e913" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_01a0355c-6e84-7e0d-809e-6a063707d501" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_01a0355c-6e84-7e0d-809e-6a063707d501" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight_01a0355c-6e84-7694-86e1-58457392c03c" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByEachWarrantOrRight_01a0355c-6e84-7694-86e1-58457392c03c" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_01a0355c-6e84-7643-b72d-0e1c02d80421" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_01a0355c-6e84-7643-b72d-0e1c02d80421" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ClassOfWarrantOrRightRedemptionPriceOfWarrantsOrRights_01a0355c-6e84-74a4-bf34-9f82b05fee85" xlink:href="oabi-20260930.xsd#oabi_ClassOfWarrantOrRightRedemptionPriceOfWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_ClassOfWarrantOrRightRedemptionPriceOfWarrantsOrRights_01a0355c-6e84-74a4-bf34-9f82b05fee85" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_RedemptionPeriod_01a0355c-6e84-7b77-8052-3514c4f75411" xlink:href="oabi-20260930.xsd#oabi_RedemptionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_RedemptionPeriod_01a0355c-6e84-7b77-8052-3514c4f75411" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_WarrantRedemptionConditionMinimumSharePrice_01a0355c-6e84-7eba-a282-9641de248b0a" xlink:href="oabi-20260930.xsd#oabi_WarrantRedemptionConditionMinimumSharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_WarrantRedemptionConditionMinimumSharePrice_01a0355c-6e84-7eba-a282-9641de248b0a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsMinimumTradingDaysPriorToRedemption_01a0355c-6e84-7cae-8286-e964da1dc071" xlink:href="oabi-20260930.xsd#oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsMinimumTradingDaysPriorToRedemption"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_ClassOfWarrantOrRightRedemptionOfWarrantsOrRightsMinimumTradingDaysPriorToRedemption_01a0355c-6e84-7cae-8286-e964da1dc071" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_01a0355c-6e84-7e7b-ac16-6ff070c02846" 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_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_01a0355c-6e84-7e7b-ac16-6ff070c02846" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesAvailableForIssuanceAnnualIncreasePercentage_01a0355c-6e84-7aac-a7c5-b1cbe8b1259e" xlink:href="oabi-20260930.xsd#oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesAvailableForIssuanceAnnualIncreasePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesAvailableForIssuanceAnnualIncreasePercentage_01a0355c-6e84-7aac-a7c5-b1cbe8b1259e" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_SaleOfStockAggregateOfferingPrice_01a0355c-6e84-7ab1-8e71-8383870ed799" xlink:href="oabi-20260930.xsd#oabi_SaleOfStockAggregateOfferingPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_SaleOfStockAggregateOfferingPrice_01a0355c-6e84-7ab1-8e71-8383870ed799" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_SaleOfStockMaximumCommissionPercentage_01a0355c-6e84-737e-b50e-248d2e3084a7" xlink:href="oabi-20260930.xsd#oabi_SaleOfStockMaximumCommissionPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_oabi_SaleOfStockMaximumCommissionPercentage_01a0355c-6e84-737e-b50e-248d2e3084a7" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_01a0355c-6e84-7050-b064-dcbf36c7ed29" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_01a0355c-6e84-7050-b064-dcbf36c7ed29" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_01a0355c-6e84-7c06-ad7d-e81600668b9c" 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_01a0355c-6e84-7c06-ad7d-e81600668b9c" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_01a0355c-6e84-703a-ab6f-bf73f166faf7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-77b3-bae7-84c6fd4fbacc" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_01a0355c-6e84-703a-ab6f-bf73f166faf7" xlink:type="arc" order="25"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/ShareBasedCompensationSummaryofShareBasedCompensationExpensebyFunctionDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#ShareBasedCompensationSummaryofShareBasedCompensationExpensebyFunctionDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/ShareBasedCompensationSummaryofShareBasedCompensationExpensebyFunctionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a0355c-6e84-7bf1-8bc8-92a7c26cb876" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a0355c-6e84-744b-b8f3-0d15aa6737a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a0355c-6e84-7bf1-8bc8-92a7c26cb876" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a0355c-6e84-744b-b8f3-0d15aa6737a4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_01a0355c-6e84-78a4-8f1c-2a04fef7a016" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a0355c-6e84-744b-b8f3-0d15aa6737a4" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_01a0355c-6e84-78a4-8f1c-2a04fef7a016" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0355c-6e84-7b57-b504-6e5db03d55cf" 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_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a0355c-6e84-7bf1-8bc8-92a7c26cb876" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0355c-6e84-7b57-b504-6e5db03d55cf" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/ShareBasedCompensationSummaryofShareBasedCompensationExpensebyFunctionDetails_1" xlink:type="simple" xlink:href="oabi-20260930.xsd#ShareBasedCompensationSummaryofShareBasedCompensationExpensebyFunctionDetails_1"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/ShareBasedCompensationSummaryofShareBasedCompensationExpensebyFunctionDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a0355c-6e84-7bf1-8bc8-92a7c26cb876" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a0355c-6e84-744b-b8f3-0d15aa6737a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a0355c-6e84-7bf1-8bc8-92a7c26cb876" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a0355c-6e84-744b-b8f3-0d15aa6737a4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0355c-6e84-7b57-b504-6e5db03d55cf" 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_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a0355c-6e84-7bf1-8bc8-92a7c26cb876" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0355c-6e84-7b57-b504-6e5db03d55cf" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/ShareBasedCompensationSummaryofShareBasedCompensationExpensebyAwardTypeDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#ShareBasedCompensationSummaryofShareBasedCompensationExpensebyAwardTypeDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/ShareBasedCompensationSummaryofShareBasedCompensationExpensebyAwardTypeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a0355c-6e84-721f-b7a7-fd165b54f70b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a0355c-6e84-708d-af2d-4767f301505a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a0355c-6e84-721f-b7a7-fd165b54f70b" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a0355c-6e84-708d-af2d-4767f301505a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a0355c-6e84-75b0-9d83-1c364e58517e" 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_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a0355c-6e84-708d-af2d-4767f301505a" xlink:to="loc_us-gaap_AwardTypeAxis_01a0355c-6e84-75b0-9d83-1c364e58517e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-75b0-9d83-1c364e58517e_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_01a0355c-6e84-75b0-9d83-1c364e58517e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-75b0-9d83-1c364e58517e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-76bf-bc0f-d691156b3aa4" 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_01a0355c-6e84-75b0-9d83-1c364e58517e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-76bf-bc0f-d691156b3aa4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_01a0355c-6e84-7d0a-b24a-4ffde353c54f" 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_01a0355c-6e84-76bf-bc0f-d691156b3aa4" xlink:to="loc_us-gaap_EmployeeStockOptionMember_01a0355c-6e84-7d0a-b24a-4ffde353c54f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0355c-6e84-70ec-bace-47933aa496bd" 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_01a0355c-6e84-76bf-bc0f-d691156b3aa4" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0355c-6e84-70ec-bace-47933aa496bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a0355c-6e84-70da-81b2-ff5692c6fc74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-76bf-bc0f-d691156b3aa4" xlink:to="loc_us-gaap_EmployeeStockMember_01a0355c-6e84-70da-81b2-ff5692c6fc74" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0355c-6e84-720e-a457-8ce29d738079" 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_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a0355c-6e84-721f-b7a7-fd165b54f70b" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0355c-6e84-720e-a457-8ce29d738079" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/ShareBasedCompensationAdditionalInformationDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#ShareBasedCompensationAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/ShareBasedCompensationAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" 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_01a0355c-6e84-7bd0-a197-c80e6734962e" 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_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0355c-6e84-7bd0-a197-c80e6734962e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_01a14562-8d79-77e6-9c25-fe4413e301f7" 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_01a0355c-6e84-7bd0-a197-c80e6734962e" xlink:to="loc_us-gaap_VestingAxis_01a14562-8d79-77e6-9c25-fe4413e301f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_01a14562-8d79-77e6-9c25-fe4413e301f7_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_01a14562-8d79-77e6-9c25-fe4413e301f7" xlink:to="loc_us-gaap_VestingDomain_01a14562-8d79-77e6-9c25-fe4413e301f7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_01a14562-8d79-78ea-bd2e-f6f40f2071b9" 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_01a14562-8d79-77e6-9c25-fe4413e301f7" xlink:to="loc_us-gaap_VestingDomain_01a14562-8d79-78ea-bd2e-f6f40f2071b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_01a14562-8d79-7f86-92f9-5bc94f8811a2" 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_01a14562-8d79-78ea-bd2e-f6f40f2071b9" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_01a14562-8d79-7f86-92f9-5bc94f8811a2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_01a14562-8d79-7e22-8e8a-edf6280bb87b" 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_01a14562-8d79-78ea-bd2e-f6f40f2071b9" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_01a14562-8d79-7e22-8e8a-edf6280bb87b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a14562-8d79-7579-a710-abb5393fb0a6" 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_01a0355c-6e84-7bd0-a197-c80e6734962e" xlink:to="loc_us-gaap_AwardTypeAxis_01a14562-8d79-7579-a710-abb5393fb0a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a14562-8d79-7579-a710-abb5393fb0a6_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_01a14562-8d79-7579-a710-abb5393fb0a6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a14562-8d79-7579-a710-abb5393fb0a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a14562-8d79-7fd8-b002-67fdb0feafdd" 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_01a14562-8d79-7579-a710-abb5393fb0a6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a14562-8d79-7fd8-b002-67fdb0feafdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_01a14562-8d79-7dfd-979e-c6eac1f742d6" 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_01a14562-8d79-7fd8-b002-67fdb0feafdd" xlink:to="loc_us-gaap_EmployeeStockOptionMember_01a14562-8d79-7dfd-979e-c6eac1f742d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a14562-e519-751b-b259-3d3f205b37ca" 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_01a14562-8d79-7fd8-b002-67fdb0feafdd" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a14562-e519-751b-b259-3d3f205b37ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a14563-17ff-7821-8626-0ac0e6e5b1e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a14562-8d79-7fd8-b002-67fdb0feafdd" xlink:to="loc_us-gaap_EmployeeStockMember_01a14563-17ff-7821-8626-0ac0e6e5b1e5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_01a14562-bf68-7324-93a7-e654cc4adb28" 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_01a0355c-6e84-7bd0-a197-c80e6734962e" xlink:to="loc_us-gaap_PlanNameAxis_01a14562-bf68-7324-93a7-e654cc4adb28" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_01a14562-bf68-7324-93a7-e654cc4adb28_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_01a14562-bf68-7324-93a7-e654cc4adb28" xlink:to="loc_us-gaap_PlanNameDomain_01a14562-bf68-7324-93a7-e654cc4adb28_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_01a14562-bf68-743b-96f7-a4dc2051b4a3" 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_01a14562-bf68-7324-93a7-e654cc4adb28" xlink:to="loc_us-gaap_PlanNameDomain_01a14562-bf68-743b-96f7-a4dc2051b4a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_OmniAbMember_01a14562-bf68-77c8-bf5d-e18c6e88b8e4" xlink:href="oabi-20260930.xsd#oabi_OmniAbMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_01a14562-bf68-743b-96f7-a4dc2051b4a3" xlink:to="loc_oabi_OmniAbMember_01a14562-bf68-77c8-bf5d-e18c6e88b8e4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_LigandMember_01a14562-bf68-7adc-908e-1052c1317345" xlink:href="oabi-20260930.xsd#oabi_LigandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_01a14562-bf68-743b-96f7-a4dc2051b4a3" xlink:to="loc_oabi_LigandMember_01a14562-bf68-7adc-908e-1052c1317345" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_01a14567-8fa1-75a4-a138-ef356e67e95f" 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_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_01a14567-8fa1-75a4-a138-ef356e67e95f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_01a14562-8d79-7914-a8de-3466beab5df1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_01a14562-8d79-7914-a8de-3466beab5df1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_01a14562-8d79-72cc-8688-c3d5fd3456c9" 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_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_01a14562-8d79-72cc-8688-c3d5fd3456c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_01a14562-8d79-757f-a4de-15d3e5e757c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_01a14562-8d79-757f-a4de-15d3e5e757c4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_01a14562-bf68-7275-997b-a63520aa400f" 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_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_01a14562-bf68-7275-997b-a63520aa400f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_01a14562-bf68-78c8-bf5e-08b9bd39ba94" 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_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_01a14562-bf68-78c8-bf5e-08b9bd39ba94" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_01a14562-bf68-7ca3-b371-5f40b376904c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_01a14562-bf68-7ca3-b371-5f40b376904c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromStockOptionsExercised_01a14562-bf68-7a62-a120-0da2621bc5a1" 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_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_ProceedsFromStockOptionsExercised_01a14562-bf68-7a62-a120-0da2621bc5a1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_01a14563-17ff-79d0-b8b7-861ca28384be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_01a14563-17ff-79d0-b8b7-861ca28384be" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_01a14563-17ff-7d56-88bb-b97d689edb55" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueVested_01a14563-17ff-7d56-88bb-b97d689edb55" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardConsecutiveOfferingPeriod_01a14563-17ff-77f4-9a77-6838814aa706" xlink:href="oabi-20260930.xsd#oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardConsecutiveOfferingPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardConsecutiveOfferingPeriod_01a14563-17ff-77f4-9a77-6838814aa706" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfPurchasePeriods_01a14563-17ff-7c7e-98cf-0edd71323722" xlink:href="oabi-20260930.xsd#oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfPurchasePeriods"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfPurchasePeriods_01a14563-17ff-7c7e-98cf-0edd71323722" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardPurchasePeriod_01a14563-17ff-7c37-ae6f-3d2886ad19e6" xlink:href="oabi-20260930.xsd#oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardPurchasePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardPurchasePeriod_01a14563-17ff-7c37-ae6f-3d2886ad19e6" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_01a14563-17ff-79ac-970d-0b2e9bc6d403" 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_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_01a14563-17ff-79ac-970d-0b2e9bc6d403" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_01a14563-38d1-7763-978e-05365d70278f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_01a14563-38d1-7763-978e-05365d70278f" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorizedPercent_01a14563-38d1-7599-8ffb-df6553dc70ca" xlink:href="oabi-20260930.xsd#oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorizedPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_oabi_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorizedPercent_01a14563-38d1-7599-8ffb-df6553dc70ca" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_01a14563-38d1-71b1-a436-409de13aac04" 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_01a0355c-6e84-71f0-b964-0e6cdb79953a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_01a14563-38d1-71b1-a436-409de13aac04" xlink:type="arc" order="16"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/ShareBasedCompensationSummaryofAssumptionsUsedtoEstimateFairValueofEmployeeStockOptionsDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#ShareBasedCompensationSummaryofAssumptionsUsedtoEstimateFairValueofEmployeeStockOptionsDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/ShareBasedCompensationSummaryofAssumptionsUsedtoEstimateFairValueofEmployeeStockOptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-74d7-8376-8382c9eed59c" 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_01a0355c-6e84-7e3b-aecd-40ed18a68c64" 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_01a0355c-6e84-74d7-8376-8382c9eed59c" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0355c-6e84-7e3b-aecd-40ed18a68c64" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a0355c-6e84-7246-b258-07f350b47d9f" 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_01a0355c-6e84-7e3b-aecd-40ed18a68c64" xlink:to="loc_us-gaap_AwardTypeAxis_01a0355c-6e84-7246-b258-07f350b47d9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-7246-b258-07f350b47d9f_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_01a0355c-6e84-7246-b258-07f350b47d9f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-7246-b258-07f350b47d9f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-73d9-966f-84ad9689d020" 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_01a0355c-6e84-7246-b258-07f350b47d9f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-73d9-966f-84ad9689d020" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_01a0355c-6e84-7d06-b622-7ec9d1e35c28" 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_01a0355c-6e84-73d9-966f-84ad9689d020" xlink:to="loc_us-gaap_EmployeeStockOptionMember_01a0355c-6e84-7d06-b622-7ec9d1e35c28" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_01a0355c-6e84-7e42-9d02-a30ccdc98f6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-74d7-8376-8382c9eed59c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_01a0355c-6e84-7e42-9d02-a30ccdc98f6d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_01a0355c-6e84-7892-9a51-0b814900acbb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-74d7-8376-8382c9eed59c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_01a0355c-6e84-7892-9a51-0b814900acbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_01a0355c-6e84-7f46-a9d6-d9018953fb96" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-74d7-8376-8382c9eed59c" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_01a0355c-6e84-7f46-a9d6-d9018953fb96" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_01a0355c-6e84-7e51-8771-3d472a6fd353" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-74d7-8376-8382c9eed59c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_01a0355c-6e84-7e51-8771-3d472a6fd353" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_01a15ebc-f932-7287-9fb4-7ab1a18500d3" 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_01a0355c-6e84-74d7-8376-8382c9eed59c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_01a15ebc-f932-7287-9fb4-7ab1a18500d3" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/ShareBasedCompensationSummaryofRSUActivityDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#ShareBasedCompensationSummaryofRSUActivityDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/ShareBasedCompensationSummaryofRSUActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0355c-6e84-7e0e-9229-d3481ff404a5" 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_01a0355c-6e84-7a18-89fc-e64361c1a522" 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_01a0355c-6e84-7e0e-9229-d3481ff404a5" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0355c-6e84-7a18-89fc-e64361c1a522" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a0355c-6e84-71b9-9094-de921d2fb4cc" 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_01a0355c-6e84-7a18-89fc-e64361c1a522" xlink:to="loc_us-gaap_AwardTypeAxis_01a0355c-6e84-71b9-9094-de921d2fb4cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-71b9-9094-de921d2fb4cc_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_01a0355c-6e84-71b9-9094-de921d2fb4cc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-71b9-9094-de921d2fb4cc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-7e4f-9206-9b46d27920a0" 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_01a0355c-6e84-71b9-9094-de921d2fb4cc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0355c-6e84-7e4f-9206-9b46d27920a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0355c-6e84-7cfe-a937-0bb8791a1b04" 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_01a0355c-6e84-7e4f-9206-9b46d27920a0" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0355c-6e84-7cfe-a937-0bb8791a1b04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a0355c-6e84-7fd0-855d-43e6fd7b5635" 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_01a0355c-6e84-7e0e-9229-d3481ff404a5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a0355c-6e84-7fd0-855d-43e6fd7b5635" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_01a0355c-6e84-7399-913e-57bab4dcb9eb" 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_01a0355c-6e84-7fd0-855d-43e6fd7b5635" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_01a0355c-6e84-7399-913e-57bab4dcb9eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a0355c-6e84-7ad1-b99a-67152187d8b2" 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_01a0355c-6e84-7fd0-855d-43e6fd7b5635" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a0355c-6e84-7ad1-b99a-67152187d8b2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_01a0355c-6e84-7a3f-91aa-18cf817cc588" 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_01a0355c-6e84-7fd0-855d-43e6fd7b5635" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_01a0355c-6e84-7a3f-91aa-18cf817cc588" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_01a0355c-6e84-70ed-9d47-49976c868551" 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_01a0355c-6e84-7fd0-855d-43e6fd7b5635" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_01a0355c-6e84-70ed-9d47-49976c868551" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_01a0355c-6e84-754f-b07e-72fc35014850" 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_01a0355c-6e84-7ca5-809c-9e15c4689b58" 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_01a0355c-6e84-7e0e-9229-d3481ff404a5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a0355c-6e84-7ca5-809c-9e15c4689b58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_01a0355c-6e84-7ccf-913b-8a6facede86b" 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_01a0355c-6e84-7ca5-809c-9e15c4689b58" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_01a0355c-6e84-7ccf-913b-8a6facede86b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_01a0355c-6e84-7bf9-b724-8f9ff51673a8" 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_01a0355c-6e84-7ca5-809c-9e15c4689b58" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_01a0355c-6e84-7bf9-b724-8f9ff51673a8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_01a0355c-6e84-728e-abc9-a4d56d22d3cf" 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_01a0355c-6e84-7ca5-809c-9e15c4689b58" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_01a0355c-6e84-728e-abc9-a4d56d22d3cf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_01a0355c-6e84-7d01-b606-7fc81971d527" 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_01a0355c-6e84-7ca5-809c-9e15c4689b58" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_01a0355c-6e84-7d01-b606-7fc81971d527" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_01a0355c-6e84-7813-87f1-5fb1787fa103" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/NetLossPerShareSummaryofCommonShareEquivalentsDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#NetLossPerShareSummaryofCommonShareEquivalentsDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/NetLossPerShareSummaryofCommonShareEquivalentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicLineItems_01a0355c-6e85-790b-a747-d85045ea2c80" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareBasicLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_01a0355c-6e85-772c-8837-863f7ca48685" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_01a0355c-6e85-790b-a747-d85045ea2c80" xlink:to="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_01a0355c-6e85-772c-8837-863f7ca48685" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a0355c-6e85-7b17-bd95-176d41d0d2d5" 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_ScheduleOfEarningsPerShareBasicByCommonClassTable_01a0355c-6e85-772c-8837-863f7ca48685" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a0355c-6e85-7b17-bd95-176d41d0d2d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0355c-6e85-7b17-bd95-176d41d0d2d5_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_01a0355c-6e85-7b17-bd95-176d41d0d2d5" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0355c-6e85-7b17-bd95-176d41d0d2d5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0355c-6e85-7ca8-96f3-3d0f936f700e" 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_01a0355c-6e85-7b17-bd95-176d41d0d2d5" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0355c-6e85-7ca8-96f3-3d0f936f700e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_01a0355c-6e85-767f-a531-f429707839d6" 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_01a0355c-6e85-7ca8-96f3-3d0f936f700e" xlink:to="loc_us-gaap_EmployeeStockOptionMember_01a0355c-6e85-767f-a531-f429707839d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_01a0355c-6e85-76f8-9f0f-37568b0b4394" 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_AntidilutiveSecuritiesNameDomain_01a0355c-6e85-7ca8-96f3-3d0f936f700e" xlink:to="loc_us-gaap_WarrantMember_01a0355c-6e85-76f8-9f0f-37568b0b4394" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0355c-6e85-7885-b6ac-ed319c5fbe5b" 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_01a0355c-6e85-7ca8-96f3-3d0f936f700e" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0355c-6e85-7885-b6ac-ed319c5fbe5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a0355c-6e85-7d06-809a-18801c18af1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0355c-6e85-7ca8-96f3-3d0f936f700e" xlink:to="loc_us-gaap_EmployeeStockMember_01a0355c-6e85-7d06-809a-18801c18af1c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0355c-6e85-78e8-a4bc-ee92af2a8400" 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_ScheduleOfEarningsPerShareBasicByCommonClassTable_01a0355c-6e85-772c-8837-863f7ca48685" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0355c-6e85-78e8-a4bc-ee92af2a8400" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e85-78e8-a4bc-ee92af2a8400_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_01a0355c-6e85-78e8-a4bc-ee92af2a8400" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e85-78e8-a4bc-ee92af2a8400_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e85-7efc-9bc4-ee3bbe320dad" 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_01a0355c-6e85-78e8-a4bc-ee92af2a8400" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e85-7efc-9bc4-ee3bbe320dad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_EarnoutSharesMember_01a0355c-6e85-7390-abfb-1fc00de3bf34" xlink:href="oabi-20260930.xsd#oabi_EarnoutSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e85-7efc-9bc4-ee3bbe320dad" xlink:to="loc_oabi_EarnoutSharesMember_01a0355c-6e85-7390-abfb-1fc00de3bf34" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_01a0355c-6e85-7c89-b1f5-cd8032c77b17" 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_ClassOfWarrantOrRightDomain_01a0355c-6e85-7efc-9bc4-ee3bbe320dad" xlink:to="loc_us-gaap_PrivatePlacementMember_01a0355c-6e85-7c89-b1f5-cd8032c77b17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_PublicWarrantsMember_01a0355c-6e85-71c7-bc8c-2ce9084ad161" xlink:href="oabi-20260930.xsd#oabi_PublicWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e85-7efc-9bc4-ee3bbe320dad" xlink:to="loc_oabi_PublicWarrantsMember_01a0355c-6e85-71c7-bc8c-2ce9084ad161" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_PIPEWarrantMember_01a0355c-6e85-7147-85ba-8f01b728571b" xlink:href="oabi-20260930.xsd#oabi_PIPEWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e85-7efc-9bc4-ee3bbe320dad" xlink:to="loc_oabi_PIPEWarrantMember_01a0355c-6e85-7147-85ba-8f01b728571b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_BackstopWarrantsMember_01a0355c-6e85-71cd-8166-8061e1bdb6b1" xlink:href="oabi-20260930.xsd#oabi_BackstopWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0355c-6e85-7efc-9bc4-ee3bbe320dad" xlink:to="loc_oabi_BackstopWarrantsMember_01a0355c-6e85-71cd-8166-8061e1bdb6b1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_01a0355c-6e85-7b7c-a700-9d431eb4dac5" 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_EarningsPerShareBasicLineItems_01a0355c-6e85-790b-a747-d85045ea2c80" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_01a0355c-6e85-7b7c-a700-9d431eb4dac5" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.omniab.com/role/SegmentInformationScheduleofSegmentReportingInformationDetails" xlink:type="simple" xlink:href="oabi-20260930.xsd#SegmentInformationScheduleofSegmentReportingInformationDetails"/>
  <link:definitionLink xlink:role="http://www.omniab.com/role/SegmentInformationScheduleofSegmentReportingInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_01a0355c-6e85-7d48-9405-d27a87210e95" 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_01a0355c-6e85-7be7-83e7-406b44e1cd99" 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_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a0355c-6e85-7be7-83e7-406b44e1cd99" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_01a0355c-6e85-7a70-95a6-24dd81ef5075" 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_01a0355c-6e85-7be7-83e7-406b44e1cd99" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_01a0355c-6e85-7a70-95a6-24dd81ef5075" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a0355c-6e85-7a70-95a6-24dd81ef5075_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_01a0355c-6e85-7a70-95a6-24dd81ef5075" xlink:to="loc_us-gaap_SegmentDomain_01a0355c-6e85-7a70-95a6-24dd81ef5075_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a0355c-6e85-7a02-87ee-8dd721fa2d70" 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_01a0355c-6e85-7a70-95a6-24dd81ef5075" xlink:to="loc_us-gaap_SegmentDomain_01a0355c-6e85-7a02-87ee-8dd721fa2d70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ReportableSegmentMember_01a0355c-6e85-74ae-8c93-cac9ebc05b5f" xlink:href="oabi-20260930.xsd#oabi_ReportableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a0355c-6e85-7a02-87ee-8dd721fa2d70" xlink:to="loc_oabi_ReportableSegmentMember_01a0355c-6e85-74ae-8c93-cac9ebc05b5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a0355c-6e85-75c3-8a5b-9f71a554ac7c" 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_01a0355c-6e85-7be7-83e7-406b44e1cd99" xlink:to="loc_srt_ProductOrServiceAxis_01a0355c-6e85-75c3-8a5b-9f71a554ac7c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a0355c-6e85-75c3-8a5b-9f71a554ac7c_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_01a0355c-6e85-75c3-8a5b-9f71a554ac7c" xlink:to="loc_srt_ProductsAndServicesDomain_01a0355c-6e85-75c3-8a5b-9f71a554ac7c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a0355c-6e85-7e1a-92c7-de7cfaca8b79" 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_01a0355c-6e85-75c3-8a5b-9f71a554ac7c" xlink:to="loc_srt_ProductsAndServicesDomain_01a0355c-6e85-7e1a-92c7-de7cfaca8b79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_XPlorationRevenueMember_01a0355c-6e85-7ae6-b41f-517e75411d3e" xlink:href="oabi-20260930.xsd#oabi_XPlorationRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a0355c-6e85-7e1a-92c7-de7cfaca8b79" xlink:to="loc_oabi_XPlorationRevenueMember_01a0355c-6e85-7ae6-b41f-517e75411d3e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a0355c-6e85-7faa-ad85-824ed542ee40" 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_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a0355c-6e85-7faa-ad85-824ed542ee40" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfRevenue_01a0355c-6e85-7b34-a104-fc9b08aba025" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_CostOfRevenue_01a0355c-6e85-7b34-a104-fc9b08aba025" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a0355c-6e85-710d-afef-4054f48d55f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a0355c-6e85-710d-afef-4054f48d55f0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ResearchAndDevelopmentExpensePersonnelRelatedExpense_01a0355c-6e85-74ee-ac32-f15791d589c4" xlink:href="oabi-20260930.xsd#oabi_ResearchAndDevelopmentExpensePersonnelRelatedExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a0355c-6e85-710d-afef-4054f48d55f0" xlink:to="loc_oabi_ResearchAndDevelopmentExpensePersonnelRelatedExpense_01a0355c-6e85-74ee-ac32-f15791d589c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ResearchAndDevelopmentExpenseExternalExpenses_01a0355c-6e85-7f20-a303-c96b6c39ef2f" xlink:href="oabi-20260930.xsd#oabi_ResearchAndDevelopmentExpenseExternalExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a0355c-6e85-710d-afef-4054f48d55f0" xlink:to="loc_oabi_ResearchAndDevelopmentExpenseExternalExpenses_01a0355c-6e85-7f20-a303-c96b6c39ef2f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_ResearchAndDevelopmentExpenseFacilityAndOtherOverheadExpenses_01a0355c-6e85-7092-b0d7-dc6e4a9ec0ee" xlink:href="oabi-20260930.xsd#oabi_ResearchAndDevelopmentExpenseFacilityAndOtherOverheadExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a0355c-6e85-710d-afef-4054f48d55f0" xlink:to="loc_oabi_ResearchAndDevelopmentExpenseFacilityAndOtherOverheadExpenses_01a0355c-6e85-7092-b0d7-dc6e4a9ec0ee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_01a0355c-6e85-7356-919b-200a4b2d10a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a0355c-6e85-710d-afef-4054f48d55f0" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_01a0355c-6e85-7356-919b-200a4b2d10a0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseAbstract_01a0355c-6e85-7104-bd5f-414a4a4f84f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GeneralAndAdministrativeExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseAbstract_01a0355c-6e85-7104-bd5f-414a4a4f84f7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_GeneralAndAdministrativeExpensePersonnelRelatedExpenses_01a0355c-6e85-7486-b414-7b34c3930a63" xlink:href="oabi-20260930.xsd#oabi_GeneralAndAdministrativeExpensePersonnelRelatedExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GeneralAndAdministrativeExpenseAbstract_01a0355c-6e85-7104-bd5f-414a4a4f84f7" xlink:to="loc_oabi_GeneralAndAdministrativeExpensePersonnelRelatedExpenses_01a0355c-6e85-7486-b414-7b34c3930a63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_GeneralAndAdministrativeExpenseExternalExpenses_01a0355c-6e85-779f-bf5c-b26920507e5d" xlink:href="oabi-20260930.xsd#oabi_GeneralAndAdministrativeExpenseExternalExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GeneralAndAdministrativeExpenseAbstract_01a0355c-6e85-7104-bd5f-414a4a4f84f7" xlink:to="loc_oabi_GeneralAndAdministrativeExpenseExternalExpenses_01a0355c-6e85-779f-bf5c-b26920507e5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_oabi_GeneralAndAdministrativeExpenseFacilityAndOtherExpenses_01a0355c-6e85-784d-a839-199dbf2782b0" xlink:href="oabi-20260930.xsd#oabi_GeneralAndAdministrativeExpenseFacilityAndOtherExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GeneralAndAdministrativeExpenseAbstract_01a0355c-6e85-7104-bd5f-414a4a4f84f7" xlink:to="loc_oabi_GeneralAndAdministrativeExpenseFacilityAndOtherExpenses_01a0355c-6e85-784d-a839-199dbf2782b0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_01a0355c-6e85-747a-937c-a905d7eeb313" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GeneralAndAdministrativeExpenseAbstract_01a0355c-6e85-7104-bd5f-414a4a4f84f7" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_01a0355c-6e85-747a-937c-a905d7eeb313" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_01a0355c-6e85-790c-8633-31938f3d5df4" 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_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_01a0355c-6e85-790c-8633-31938f3d5df4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherOperatingIncomeExpenseNet_01a0355c-6e85-739c-9239-706967be8cfd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherOperatingIncomeExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_OtherOperatingIncomeExpenseNet_01a0355c-6e85-739c-9239-706967be8cfd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_01a0355c-6e85-7465-baf1-4c548798d482" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_OperatingExpenses_01a0355c-6e85-7465-baf1-4c548798d482" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingOtherItemAmount_01a0355c-6e85-72de-8f8e-64deb8f71010" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingOtherItemAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_SegmentReportingOtherItemAmount_01a0355c-6e85-72de-8f8e-64deb8f71010" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_01a0355c-6e85-7063-bb20-57112bf0ad1f" 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_SegmentReportingInformationLineItems_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_01a0355c-6e85-7063-bb20-57112bf0ad1f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_01a0355c-6e85-705a-9606-de743aafa0cb" 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_SegmentReportingInformationLineItems_01a0355c-6e85-7d48-9405-d27a87210e95" xlink:to="loc_us-gaap_NetIncomeLoss_01a0355c-6e85-705a-9606-de743aafa0cb" xlink:type="arc" order="9"/>
  </link:definitionLink>
</link:linkbase>
