<?xml version="1.0" encoding="ASCII"?>
<!-- XBRL document created with Praxa -->
<!-- Copyright (c) 2026 Praxa LLC. All Rights Reserved.-->
<!-- Creation Date: 2026-08-13T16:10:26.2694+00:00 -->
<xs:schema targetNamespace="http://onconetix.com/20260630" attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:onco="http://onconetix.com/20260630" xmlns:dei="http://xbrl.sec.gov/dei/2026" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:dtr="http://www.xbrl.org/dtr/type/2024-01-31" xmlns:us-gaap="http://fasb.org/us-gaap/2026" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2024-01-31" xmlns:srt="http://fasb.org/srt/2026" xmlns:country="http://xbrl.sec.gov/country/2026" xmlns:enum2="http://xbrl.org/2020/extensible-enumerations-2.0" xmlns:ecd-sub="http://xbrl.sec.gov/ecd-sub/2026">
  <xs:import namespace="http://xbrl.sec.gov/dei/2026" schemaLocation="https://xbrl.sec.gov/dei/2026/dei-2026.xsd" />
  <xs:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" />
  <xs:import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd" />
  <xs:import namespace="http://www.xbrl.org/dtr/type/2024-01-31" schemaLocation="https://www.xbrl.org/dtr/type/2024-01-31/types.xsd" />
  <xs:import namespace="http://fasb.org/us-gaap/2026" schemaLocation="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd" />
  <xs:import namespace="http://fasb.org/srt/2026" schemaLocation="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd" />
  <xs:import namespace="http://xbrl.sec.gov/country/2026" schemaLocation="https://xbrl.sec.gov/country/2026/country-2026.xsd" />
  <xs:import namespace="http://xbrl.org/2020/extensible-enumerations-2.0" schemaLocation="https://www.xbrl.org/2020/extensible-enumerations-2.0.xsd" />
  <xs:import namespace="http://xbrl.sec.gov/ecd-sub/2026" schemaLocation="https://xbrl.sec.gov/ecd/2026/ecd-sub-2026.xsd" />
  <xs:annotation>
    <xs:appinfo>
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="onco-20260630_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:title="Presentation link" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="onco-20260630_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:title="Label link" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="onco-20260630_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:title="Definition link" xlink:type="simple" />
      <link:linkbaseRef xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="onco-20260630_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:title="Calculation link" xlink:type="simple" />
      <link:roleType roleURI="http://onconetix.com/20260630/role/Cover" id="Cover">
        <link:definition>995300 - Document - Cover</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/CondensedConsolidatedBalanceSheets" id="CondensedConsolidatedBalanceSheets">
        <link:definition>995301 - Statement - Condensed Consolidated Balance Sheets</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/CondensedConsolidatedBalanceSheetsParentheticals" id="CondensedConsolidatedBalanceSheetsParentheticals">
        <link:definition>995302 - Statement - Condensed Consolidated Balance Sheets (Parentheticals)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/CondensedConsolidatedStatementsofOperationsandComprehensiveLossUnaudited" id="CondensedConsolidatedStatementsofOperationsandComprehensiveLossUnaudited">
        <link:definition>995303 - Statement - Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/StockholdersEquityDeficitUnaudited" id="StockholdersEquityDeficitUnaudited">
        <link:definition>995304 - Statement - Stockholders&#8217; Equity (Deficit) (Unaudited)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/ConsolidatedStatementsofCashFlowsUnaudited" id="ConsolidatedStatementsofCashFlowsUnaudited">
        <link:definition>995305 - Statement - Consolidated Statements of Cash Flows (Unaudited)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/OrganizationandBasisofPresentation" id="OrganizationandBasisofPresentation">
        <link:definition>995306 - Disclosure - Organization and Basis of Presentation</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/GoingConcernandManagementsPlans" id="GoingConcernandManagementsPlans">
        <link:definition>995307 - Disclosure - Going Concern and Management&#8217;s Plans</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SummaryofSignificantAccountingPolicies" id="SummaryofSignificantAccountingPolicies">
        <link:definition>995308 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/BalanceSheetDetails" id="BalanceSheetDetails">
        <link:definition>995309 - Disclosure - Balance Sheet Details</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SignificantAgreements" id="SignificantAgreements">
        <link:definition>995310 - Disclosure - Significant Agreements</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/NotesPayable" id="NotesPayable">
        <link:definition>995311 - Disclosure - Notes Payable</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SubscriptionAgreement" id="SubscriptionAgreement">
        <link:definition>995312 - Disclosure - Subscription Agreement</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/WarrantandDerivativeLiabilities" id="WarrantandDerivativeLiabilities">
        <link:definition>995313 - Disclosure - Warrant and Derivative Liabilities</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/ConvertiblePreferredStockandStockholdersEquity" id="ConvertiblePreferredStockandStockholdersEquity">
        <link:definition>995314 - Disclosure - Convertible Preferred Stock and Stockholders&#8217; Equity</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/CommitmentsandContingencies" id="CommitmentsandContingencies">
        <link:definition>995315 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/RelatedPartyTransactions" id="RelatedPartyTransactions">
        <link:definition>995316 - Disclosure - Related Party Transactions</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/NetLossPerShare" id="NetLossPerShare">
        <link:definition>995317 - Disclosure - Net Loss Per Share</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SegmentInformation" id="SegmentInformation">
        <link:definition>995318 - Disclosure - Segment Information</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SubsequentEvents" id="SubsequentEvents">
        <link:definition>995319 - Disclosure - Subsequent Events</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/AccountingPoliciesbyPolicyPolicies" id="AccountingPoliciesbyPolicyPolicies">
        <link:definition>996320 - Disclosure - Accounting Policies, by Policy (Policies)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SummaryofSignificantAccountingPoliciesTables" id="SummaryofSignificantAccountingPoliciesTables">
        <link:definition>996321 - Disclosure - Summary of Significant Accounting Policies (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/BalanceSheetDetailsTables" id="BalanceSheetDetailsTables">
        <link:definition>996322 - Disclosure - Balance Sheet Details (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/WarrantandDerivativeLiabilitiesTables" id="WarrantandDerivativeLiabilitiesTables">
        <link:definition>996323 - Disclosure - Warrant and Derivative Liabilities (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/ConvertiblePreferredStockandStockholdersEquityTables" id="ConvertiblePreferredStockandStockholdersEquityTables">
        <link:definition>996324 - Disclosure - Convertible Preferred Stock and Stockholders&#8217; Equity (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/NetLossPerShareTables" id="NetLossPerShareTables">
        <link:definition>996325 - Disclosure - Net Loss Per Share (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SegmentInformationTables" id="SegmentInformationTables">
        <link:definition>996326 - Disclosure - Segment Information (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/OrganizationandBasisofPresentationDetails" id="OrganizationandBasisofPresentationDetails">
        <link:definition>996327 - Disclosure - Organization and Basis of Presentation (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/GoingConcernandManagementsPlansDetails" id="GoingConcernandManagementsPlansDetails">
        <link:definition>996328 - Disclosure - Going Concern and Management&#8217;s Plans (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SummaryofSignificantAccountingPoliciesScheduleofFairValueofFinancialInstrumentsMeasuredonaRecurringBasisDetails" id="SummaryofSignificantAccountingPoliciesScheduleofFairValueofFinancialInstrumentsMeasuredonaRecurringBasisDetails">
        <link:definition>996329 - Disclosure - Summary of Significant Accounting Policies - Schedule of Fair Value of Financial Instruments Measured on a Recurring Basis (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/BalanceSheetDetailsDetails" id="BalanceSheetDetailsDetails">
        <link:definition>996330 - Disclosure - Balance Sheet Details (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/BalanceSheetDetailsScheduleofInventoriesDetails" id="BalanceSheetDetailsScheduleofInventoriesDetails">
        <link:definition>996331 - Disclosure - Balance Sheet Details - Schedule of Inventories (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/BalanceSheetDetailsScheduleofPrepaidExpensesandOtherCurrentAssetsDetails" id="BalanceSheetDetailsScheduleofPrepaidExpensesandOtherCurrentAssetsDetails">
        <link:definition>996332 - Disclosure - Balance Sheet Details - Schedule of Prepaid Expenses and Other Current Assets (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/BalanceSheetDetailsScheduleofGoodwillDetails" id="BalanceSheetDetailsScheduleofGoodwillDetails">
        <link:definition>996333 - Disclosure - Balance Sheet Details - Schedule of Goodwill (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/BalanceSheetDetailsScheduleofAccruedExpensesDetails" id="BalanceSheetDetailsScheduleofAccruedExpensesDetails">
        <link:definition>996334 - Disclosure - Balance Sheet Details - Schedule of Accrued Expenses (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SignificantAgreementsDetails" id="SignificantAgreementsDetails">
        <link:definition>996335 - Disclosure - Significant Agreements (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/NotesPayableDetails" id="NotesPayableDetails">
        <link:definition>996336 - Disclosure - Notes Payable (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SubscriptionAgreementDetails" id="SubscriptionAgreementDetails">
        <link:definition>996337 - Disclosure - Subscription Agreement (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/WarrantandDerivativeLiabilitiesScheduleofActivityfortheContingentWarrantLiabilitiesUsingUnobservableLevel3InputsDetails" id="WarrantandDerivativeLiabilitiesScheduleofActivityfortheContingentWarrantLiabilitiesUsingUnobservableLevel3InputsDetails">
        <link:definition>996338 - Disclosure - Warrant and Derivative Liabilities - Schedule of Activity for the Contingent Warrant Liabilities, Using Unobservable Level 3 Inputs (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/WarrantandDerivativeLiabilitiesScheduleofInputsUsedtoDeterminetheFairValueoftheDerivativeandWarrantLiabilitiesDetails" id="WarrantandDerivativeLiabilitiesScheduleofInputsUsedtoDeterminetheFairValueoftheDerivativeandWarrantLiabilitiesDetails">
        <link:definition>996339 - Disclosure - Warrant and Derivative Liabilities - Schedule of Inputs Used to Determine the Fair Value of the Derivative and Warrant Liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/WarrantandDerivativeLiabilitiesScheduleofCompanysDerivativeLiabilitiesandWarrantLiabilitiesthatareMeasuredatFairValueonaRecurringBasisDetails" id="WarrantandDerivativeLiabilitiesScheduleofCompanysDerivativeLiabilitiesandWarrantLiabilitiesthatareMeasuredatFairValueonaRecurringBasisDetails">
        <link:definition>996340 - Disclosure - Warrant and Derivative Liabilities - Schedule of Company&#8217;s Derivative Liabilities and Warrant Liabilities that are Measured at Fair Value on a Recurring Basis (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/WarrantandDerivativeLiabilitiesScheduleofChangeintheFairValueoftheDerivativeLiabilitiesandWarrantLiabilitiesDetails" id="WarrantandDerivativeLiabilitiesScheduleofChangeintheFairValueoftheDerivativeLiabilitiesandWarrantLiabilitiesDetails">
        <link:definition>996341 - Disclosure - Warrant and Derivative Liabilities - Schedule of Change in the Fair Value of the Derivative Liabilities and Warrant Liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/ConvertiblePreferredStockandStockholdersEquityDetails" id="ConvertiblePreferredStockandStockholdersEquityDetails">
        <link:definition>996342 - Disclosure - Convertible Preferred Stock and Stockholders&#8217; Equity - (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/ConvertiblePreferredStockandStockholdersEquityPart2Details" id="ConvertiblePreferredStockandStockholdersEquityPart2Details">
        <link:definition>996343 - Disclosure - Convertible Preferred Stock and Stockholders&#8217; Equity - Part 2 (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/ConvertiblePreferredStockandStockholdersEquityPart3Details" id="ConvertiblePreferredStockandStockholdersEquityPart3Details">
        <link:definition>996344 - Disclosure - Convertible Preferred Stock and Stockholders&#8217; Equity - Part 3 (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/ConvertiblePreferredStockandStockholdersEquityScheduleofOutstandingWarrantsDetails" id="ConvertiblePreferredStockandStockholdersEquityScheduleofOutstandingWarrantsDetails">
        <link:definition>996345 - Disclosure - Convertible Preferred Stock and Stockholders&#8217; Equity - Schedule of Outstanding Warrants (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/ConvertiblePreferredStockandStockholdersEquityScheduleofRestrictedStockDetails" id="ConvertiblePreferredStockandStockholdersEquityScheduleofRestrictedStockDetails">
        <link:definition>996346 - Disclosure - Convertible Preferred Stock and Stockholders&#8217; Equity - Schedule of Restricted Stock (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/ConvertiblePreferredStockandStockholdersEquityScheduleofStockBasedCompensationExpenseDetails" id="ConvertiblePreferredStockandStockholdersEquityScheduleofStockBasedCompensationExpenseDetails">
        <link:definition>996347 - Disclosure - Convertible Preferred Stock and Stockholders&#8217; Equity - Schedule of Stock-Based Compensation Expense (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/CommitmentsandContingenciesDetails" id="CommitmentsandContingenciesDetails">
        <link:definition>996348 - Disclosure - Commitments and Contingencies (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/RelatedPartyTransactionsDetails" id="RelatedPartyTransactionsDetails">
        <link:definition>996349 - Disclosure - Related Party Transactions (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/NetLossPerShareScheduleofComputationofDilutedSharesOutstandingDetails" id="NetLossPerShareScheduleofComputationofDilutedSharesOutstandingDetails">
        <link:definition>996350 - Disclosure - Net Loss Per Share - Schedule of Computation of Diluted Shares Outstanding (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SegmentInformationDetails" id="SegmentInformationDetails">
        <link:definition>996351 - Disclosure - Segment Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SegmentInformationScheduleofRevenuebyGeographicalAreaDetails" id="SegmentInformationScheduleofRevenuebyGeographicalAreaDetails">
        <link:definition>996352 - Disclosure - Segment Information - Schedule of Revenue by Geographical Area (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType roleURI="http://onconetix.com/20260630/role/SubsequentEventsDetails" id="SubsequentEventsDetails">
        <link:definition>996353 - Disclosure - Subsequent Events (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xs:appinfo>
  </xs:annotation>
  <xs:element id="onco_SeriesCRedeemablePreferredStockMember" name="SeriesCRedeemablePreferredStockMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_DeemedDividendSeriesCPreferredStock" name="DeemedDividendSeriesCPreferredStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_GainLossesOnForgivenessOfAccountsPayable" name="GainLossesOnForgivenessOfAccountsPayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_StockIssuedDuringPeriodValueIssuanceOfCommonStockInConnectionWithTheELOC" name="StockIssuedDuringPeriodValueIssuanceOfCommonStockInConnectionWithTheELOC" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_StockIssuedDurngPeriodSharesIssuanceOfCommonStockInConnectionWithTheELOC" name="StockIssuedDurngPeriodSharesIssuanceOfCommonStockInConnectionWithTheELOC" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_DeemedDividendOwedToSeriesCPreferredStockShareholders" name="DeemedDividendOwedToSeriesCPreferredStockShareholders" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_StockIssuedDuringPeriodValueConversionOfSeriesDPreferredStockToCommonStock" name="StockIssuedDuringPeriodValueConversionOfSeriesDPreferredStockToCommonStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_DampOInsurancePremiumFinanced" name="DampOInsurancePremiumFinanced" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_ConversionOfSeriesDPreferredStockToCommonStock" name="ConversionOfSeriesDPreferredStockToCommonStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ConversionOfSeriesEPreferredStockToCommonStock" name="ConversionOfSeriesEPreferredStockToCommonStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ChangeInFairValueOfSubscriptionAgreementLiabilityRelatedParty" name="ChangeInFairValueOfSubscriptionAgreementLiabilityRelatedParty" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ProbabilityOfSurvivalMember" name="ProbabilityOfSurvivalMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ExchangeOfPreferredStock" name="ExchangeOfPreferredStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_SecuritiesPurchaseAgreementMember" name="SecuritiesPurchaseAgreementMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesDConvertiblePreferredStockMember" name="SeriesDConvertiblePreferredStockMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesDWarrantsMember" name="SeriesDWarrantsMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ProceedsFromNetCash" name="ProceedsFromNetCash" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_StockCombinationsOfFloorPrice" name="StockCombinationsOfFloorPrice" type="dtr:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_SeriesEConvertiblePreferredStockMember" name="SeriesEConvertiblePreferredStockMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_InsuranceFinancingMember" name="InsuranceFinancingMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ScheduleOfChangeInTheFairValueOfTheDerivativeLiabilitiesAndWarrantLiabilitiesAbstract" name="ScheduleOfChangeInTheFairValueOfTheDerivativeLiabilitiesAndWarrantLiabilitiesAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_WarrantandDerivativeLiabilitiesScheduleofChangeintheFairValueoftheDerivativeLiabilitiesandWarrantLiabilitiesDetailsTable" name="WarrantandDerivativeLiabilitiesScheduleofChangeintheFairValueoftheDerivativeLiabilitiesandWarrantLiabilitiesDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_DerivativeLiabilitiesMember" name="DerivativeLiabilitiesMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_WarrantLiabilitiesMember" name="WarrantLiabilitiesMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_WarrantandDerivativeLiabilitiesScheduleofChangeintheFairValueoftheDerivativeLiabilitiesandWarrantLiabilitiesDetailsLineItems" name="WarrantandDerivativeLiabilitiesScheduleofChangeintheFairValueoftheDerivativeLiabilitiesandWarrantLiabilitiesDetailsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWarrantsVestedAndExercisable" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWarrantsVestedAndExercisable" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xs:element id="onco_ClosingScenarioMember" name="ClosingScenarioMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_NonclosingScenarioMember" name="NonclosingScenarioMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ScheduleOfFairValueOfFinancialInstrumentsMeasuredOnARecurringBasisAbstract" name="ScheduleOfFairValueOfFinancialInstrumentsMeasuredOnARecurringBasisAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesDDerivativeLiabilitiesMember" name="SeriesDDerivativeLiabilitiesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_GainOrLossInFairValueOfTheRelatedPartySubscriptionLiability" name="GainOrLossInFairValueOfTheRelatedPartySubscriptionLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ConversionsOfDerivativeLiabilities" name="ConversionsOfDerivativeLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_SubscriptionAgreementMember" name="SubscriptionAgreementMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SubscriptionAgreementLiabilityMember" name="SubscriptionAgreementLiabilityMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SubscriptionAgreementUnits" name="SubscriptionAgreementUnits" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_SubscriptionAgreementLiabilityNoncurrent" name="SubscriptionAgreementLiabilityNoncurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ChangeInFairValueOfSubscriptionAgreementLiability" name="ChangeInFairValueOfSubscriptionAgreementLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ConvertiblePreferredStockandStockholdersEquityPart1DetailsTable" name="ConvertiblePreferredStockandStockholdersEquityPart1DetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesAConvertiblePreferredStockMember" name="SeriesAConvertiblePreferredStockMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_VeruIncMember" name="VeruIncMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ConvertiblePreferredStockandStockholdersEquityPart3DetailsTable" name="ConvertiblePreferredStockandStockholdersEquityPart3DetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SummaryofSignificantAccountingPoliciesScheduleofFairValueofFinancialInstrumentsMeasuredonaRecurringBasisDetailsTable" name="SummaryofSignificantAccountingPoliciesScheduleofFairValueofFinancialInstrumentsMeasuredonaRecurringBasisDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesBConvertiblePreferredStockMember" name="SeriesBConvertiblePreferredStockMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesCConvertiblePreferredStockMember" name="SeriesCConvertiblePreferredStockMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesDDerivativeLiabilitiesAndWarrantLiabilitiesMember" name="SeriesDDerivativeLiabilitiesAndWarrantLiabilitiesMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesEDerivativeLiabilitiesAndWarrantLiabilitiesMember" name="SeriesEDerivativeLiabilitiesAndWarrantLiabilitiesMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ScheduleOfCompanySDerivativeLiabilitiesAndWarrantLiabilitiesThatAreMeasuredAtFairValueOnARecurringBasisAbstract" name="ScheduleOfCompanySDerivativeLiabilitiesAndWarrantLiabilitiesThatAreMeasuredAtFairValueOnARecurringBasisAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_WarrantandDerivativeLiabilitiesScheduleofCompanysDerivativeLiabilitiesandWarrantLiabilitiesthatareMeasuredatFairValueonaRecurringBasisDetailsTable" name="WarrantandDerivativeLiabilitiesScheduleofCompanysDerivativeLiabilitiesandWarrantLiabilitiesthatareMeasuredatFairValueonaRecurringBasisDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_OnconetixEquityIncentivePlansMember" name="OnconetixEquityIncentivePlansMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_TwoThousandTwentyTwoPlanMember" name="TwoThousandTwentyTwoPlanMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_PMXOptionPlanMember" name="PMXOptionPlanMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ContingentWarrantLiabilities" name="ContingentWarrantLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_OutstandingStockOptions" name="OutstandingStockOptions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ScheduleOfActivityForTheContingentWarrantLiabilitiesUsingUnobservableLevel3InputsAbstract" name="ScheduleOfActivityForTheContingentWarrantLiabilitiesUsingUnobservableLevel3InputsAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_OrganizationandBasisofPresentationDetailsTable" name="OrganizationandBasisofPresentationDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_OnconetixIncMember" name="OnconetixIncMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ProteomedixMember" name="ProteomedixMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_GoingConcernAndManagementsPlansAbstract" name="GoingConcernAndManagementsPlansAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_GoingConcernandManagementsPlansDetailsTable" name="GoingConcernandManagementsPlansDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SignificantAgreementsAbstract" name="SignificantAgreementsAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SignificantAgreementsDetailsTable" name="SignificantAgreementsDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_LicenseAgreementMember" name="LicenseAgreementMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesFPIPEFinancingMember" name="SeriesFPIPEFinancingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_JulyTwoThousandTwentySixELOCPurchaseAgreementMember" name="JulyTwoThousandTwentySixELOCPurchaseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_CommonSharesPercentage" name="CommonSharesPercentage" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_AggregatePurchasePrice" name="AggregatePurchasePrice" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_DrMeierMember" name="DrMeierMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SuccessFeesPercentage" name="SuccessFeesPercentage" type="dtr:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_EarnedByTheAffiliatedFirm" name="EarnedByTheAffiliatedFirm" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_NumberOfBusinessSegment" name="NumberOfBusinessSegment" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_LocationStatementOfIncomeAxis" name="LocationStatementOfIncomeAxis" type="xbrli:stringItemType" substitutionGroup="xbrldt:dimensionItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_LocationStatementOfIncomeDomain" name="LocationStatementOfIncomeDomain" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SellingGeneralAndAdministrativeMember" name="SellingGeneralAndAdministrativeMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ResearchAndDevelopmentMember" name="ResearchAndDevelopmentMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_RelatedPartyTransactionsDetailsTable" name="RelatedPartyTransactionsDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ScheduleOfRestrictedStockAbstract" name="ScheduleOfRestrictedStockAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ConvertiblePreferredStockandStockholdersEquityScheduleofRestrictedStockDetailsTable" name="ConvertiblePreferredStockandStockholdersEquityScheduleofRestrictedStockDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_DueFromShareholdersMember" name="DueFromShareholdersMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_CashInLieuOfShares" name="CashInLieuOfShares" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_StockIssuedDuringPeriodShareSettlementOfSubscriptionAgreementLiability" name="StockIssuedDuringPeriodShareSettlementOfSubscriptionAgreementLiability" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_StockIssuedDuringPeriodValueSettlementOfSubscriptionAgreementLiability" name="StockIssuedDuringPeriodValueSettlementOfSubscriptionAgreementLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_SettlementOfSubscriptionAgreementLiability" name="SettlementOfSubscriptionAgreementLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ChangeInFairValueOfContingentWarrantLiabilities" name="ChangeInFairValueOfContingentWarrantLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_ChangeInFairValueOfDerivativeLiabilities" name="ChangeInFairValueOfDerivativeLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_ChangeInFairValueOfSeriesEDerivativeLiability" name="ChangeInFairValueOfSeriesEDerivativeLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_StockIssuedDuringPeriodSharesConversionOfSeriesDPreferredStockToCommonStock" name="StockIssuedDuringPeriodSharesConversionOfSeriesDPreferredStockToCommonStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_GainOnForgivenessOfAccountsPayable" name="GainOnForgivenessOfAccountsPayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ChangeInFairValueOfSeriesDDerivativeLiability" name="ChangeInFairValueOfSeriesDDerivativeLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_SubscriptionAgreementAbstract" name="SubscriptionAgreementAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SubscriptionAgreementDisclosureTextBlock" name="SubscriptionAgreementDisclosureTextBlock" type="dtr:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_SignificantAgreementsTextBlock" name="SignificantAgreementsTextBlock" type="dtr:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_ResearchAndDevelopmentExpenses" name="ResearchAndDevelopmentExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_ScheduleOfInputsUsedToDetermineTheFairValueOfTheDerivativeAndWarrantLiabilitiesAbstract" name="ScheduleOfInputsUsedToDetermineTheFairValueOfTheDerivativeAndWarrantLiabilitiesAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ProbabilityMember" name="ProbabilityMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ChangeInFairValueAdjustmentsOfWarrantsLiability" name="ChangeInFairValueAdjustmentsOfWarrantsLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_ScheduleOfOutstandingWarrantsAbstract" name="ScheduleOfOutstandingWarrantsAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice" type="dtr:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestExercisableWeightedAverageRemainingContractualTerm1" name="SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndExpectedToVestExercisableWeightedAverageRemainingContractualTerm1" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercised" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisedInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisedInPeriod" type="dtr:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_ContingentWarrantLiabilityMember" name="ContingentWarrantLiabilityMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesDAndEDerivativeLiabilitiesMember" name="SeriesDAndEDerivativeLiabilitiesMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesCPIPEWarrantsMember" name="SeriesCPIPEWarrantsMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_BalanceSheetDetailsDetailsTable" name="BalanceSheetDetailsDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_NumberOfDesignatedShares" name="NumberOfDesignatedShares" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xs:element id="onco_ConvertiblePreferredStockandStockholdersEquityPart2DetailsTable" name="ConvertiblePreferredStockandStockholdersEquityPart2DetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ELOCMember" name="ELOCMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SeriesEPreferredStockPIPESecuritiesMember" name="SeriesEPreferredStockPIPESecuritiesMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_MaximumSaleOfNewlyIssuedCommonStock" name="MaximumSaleOfNewlyIssuedCommonStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_SharesOfCommonStockValue" name="SharesOfCommonStockValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ScheduleOfChangeInTheFairValueOfTheDerivativeLiabilitiesAndWarrantLiabilitiesTableTextBlock" name="ScheduleOfChangeInTheFairValueOfTheDerivativeLiabilitiesAndWarrantLiabilitiesTableTextBlock" type="dtr:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_GoingConcernAndManagementsPlansLineItems" name="GoingConcernAndManagementsPlansLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_WorkingCapitalDeficit" name="WorkingCapitalDeficit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_ImmunoviaABMember" name="ImmunoviaABMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_NonRefundableUpfrontLicenseFee" name="NonRefundableUpfrontLicenseFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xs:element id="onco_NotesPayableDetailsTable" name="NotesPayableDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SubscriptionAgreementDetailsTable" name="SubscriptionAgreementDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_UnitsIssuedDuringPeriodSharesNewIssues" name="UnitsIssuedDuringPeriodSharesNewIssues" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_UnitsIssuedDuringPeriodWarrantsPreFunded" name="UnitsIssuedDuringPeriodWarrantsPreFunded" type="dtr:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_ProvisionForInterestDebenture" name="ProvisionForInterestDebenture" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xs:element id="onco_InterestRatePercentage" name="InterestRatePercentage" type="dtr:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_SeriesSeedPreferredStockMember" name="SeriesSeedPreferredStockMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_ELOCPurchaseAgreementMember" name="ELOCPurchaseAgreementMember" type="dtr:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_PercentageOfCommonStockOutstanding" name="PercentageOfCommonStockOutstanding" type="dtr:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_GrossProceedsFromSaleOfCommonStock" name="GrossProceedsFromSaleOfCommonStock" type="dtr:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_TradingDays" name="TradingDays" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_SeriesDAndEWarrantsMember" name="SeriesDAndEWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_IncrementalCommonStockIssuableUponConversionOfSeriesCRedeemablePreferredStock" name="IncrementalCommonStockIssuableUponConversionOfSeriesCRedeemablePreferredStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_ScheduleOfStockBasedCompensationExpenseAbstract" name="ScheduleOfStockBasedCompensationExpenseAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_SubsequentEventsDetailsTable" name="SubsequentEventsDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xs:element id="onco_IncrementalCommonSharesAttributableToConversionOfSeriesDPreferredStock" name="IncrementalCommonSharesAttributableToConversionOfSeriesDPreferredStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_IncrementalCommonStockIssuableUponConversionOfSeriesERedeemablePreferredStock" name="IncrementalCommonStockIssuableUponConversionOfSeriesERedeemablePreferredStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xs:element id="onco_ScheduleOfRevenueByGeographicalAreaAbstract" name="ScheduleOfRevenueByGeographicalAreaAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xs:schema>
