

<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2019-06-04T22:36:24.7569663+00:00 -->
<!-- Version            : 3.5.7075.24306 -->
<!-- Package ID         : a39eaafc1c6c4fedbec7a1a0dd222aba -->
<!-- Copyright (c) 2019 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="oesx_MinimumSubscriptionPercentage" name="MinimumSubscriptionPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_RightsDistributionDateTriggerNumberOfBusinessDaysAfterCommencementOrOfferOfAcquisition" name="RightsDistributionDateTriggerNumberOfBusinessDaysAfterCommencementOrOfferOfAcquisition" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_AcquisitionShareVoidPercentage" name="AcquisitionShareVoidPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_SharePurchaseRightsRedemptionPrice" name="SharePurchaseRightsRedemptionPrice" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="oesx_EmployeeStockPurchasePlansSharesAuthorized" name="EmployeeStockPurchasePlansSharesAuthorized" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="oesx_MaximumAmountLimitForEmployeeStockPurchasePlanPerEmployee" name="MaximumAmountLimitForEmployeeStockPurchasePlanPerEmployee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_EmployeeStockPurchasePlanPurchasePriceToMarketPriceMatchingPercentage" name="EmployeeStockPurchasePlanPurchasePriceToMarketPriceMatchingPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="oesx_EmployeeStockPurchasePlanClosingMarketPriceOfSharesPurchased" name="EmployeeStockPurchasePlanClosingMarketPriceOfSharesPurchased" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_EmployeeStockPurchasePlansDebtStructureSharesIssued" name="EmployeeStockPurchasePlansDebtStructureSharesIssued" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_EmployeeStockPurchasePlansDebtStructureDirectLoanAmount" name="EmployeeStockPurchasePlansDebtStructureDirectLoanAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="oesx_EmployeeStockPurchasePlansDebtStructureDirectLoanRepaymentAmount" name="EmployeeStockPurchasePlansDebtStructureDirectLoanRepaymentAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_SummaryOfIssuanceOfStockUnderEmployeeStockOwnershipPlanAbstract" name="SummaryOfIssuanceOfStockUnderEmployeeStockOwnershipPlanAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_NonEmployeeDirectorMember" name="NonEmployeeDirectorMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_TwoThousandFourStockAndIncentiveAndAwardPlanMember" name="TwoThousandFourStockAndIncentiveAndAwardPlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerSharePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardPerSharePrice" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_FairValveOfClosingCommonStockPrice" name="FairValveOfClosingCommonStockPrice" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="oesx_NumberOfRecentlyRetiredMembersOfBoard" name="NumberOfRecentlyRetiredMembersOfBoard" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_CostOfProductsMember" name="CostOfProductsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_CostOfServicesMember" name="CostOfServicesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_FairValueOfClosingCommonStockPrice" name="FairValueOfClosingCommonStockPrice" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="oesx_RangeTwoMember" name="RangeTwoMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_RangeThreeMember" name="RangeThreeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_RangeFourMember" name="RangeFourMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_RangeFiveMember" name="RangeFiveMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_RangeSixMember" name="RangeSixMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedNextTwelveMonths" name="EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedNextTwelveMonths" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueInTwoYears" name="EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueInTwoYears" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueInThreeYears" name="EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueInThreeYears" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueInFourYears" name="EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueInFourYears" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueInFiveYears" name="EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueInFiveYears" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueThereafter" name="EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueThereafter" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_USMarketsMember" name="USMarketsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_EngineeredSystemsMember" name="EngineeredSystemsMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_DistributionServicesMember" name="DistributionServicesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_PaymentsToAcquirePropertyPlantAndEquipmentAndProductiveAssets" name="PaymentsToAcquirePropertyPlantAndEquipmentAndProductiveAssets" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="oesx_FirstAmendmentMember" name="FirstAmendmentMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>





