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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2022-08-08T16:42:56.7141743+00:00 -->
<!-- Version            : 5.0.1.747 -->
<!-- Package ID         : 4fc44d86180b4cdfb2d650485e0d2cd0 -->
<!-- Copyright (c) 2022 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="onto_SystemsAndSoftwareRevenueMember" name="SystemsAndSoftwareRevenueMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="onto_PartsRevenueMember" name="PartsRevenueMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="onto_ServiceRevenueMember" name="ServiceRevenueMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="onto_GeographicReportingAbstract" name="GeographicReportingAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="onto_SoutheastAsiaMember" name="SoutheastAsiaMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="onto_SKHynixIncMember" name="SKHynixIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="onto_TaiwanSemiconductorManufacturingCompanyLimitedMember" name="TaiwanSemiconductorManufacturingCompanyLimitedMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="onto_SamsungSemiconductorMember" name="SamsungSemiconductorMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="onto_SharesRepurchaseAuthorizationLineItems" name="SharesRepurchaseAuthorizationLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="onto_SharesRepurchaseAuthorizationTable" name="SharesRepurchaseAuthorizationTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
