<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2022-06-10T15:08:07.2575527+00:00 -->
<!-- Version            : 5.0.1.747 -->
<!-- Package ID         : f7e6fc8338df4142a10081eec4b30e5e -->
<!-- Copyright (c) 2022 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="oesx_DebtInstrumentOneMember" name="DebtInstrumentOneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_DebtInstrumentTwoMember" name="DebtInstrumentTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_LineOfCreditSpringingMinimumFixedCostCoverageRatio" name="LineOfCreditSpringingMinimumFixedCostCoverageRatio" type="xbrli:pureItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="oesx_SpringingMinimumFixedCostCoverageRatioCovenantAmountOfExcessAvailabilityUnderCreditFacility" name="SpringingMinimumFixedCostCoverageRatioCovenantAmountOfExcessAvailabilityUnderCreditFacility" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="oesx_SpringingMinimumFixedCostCoverageRatioCovenantPercentageOfCommittedFacility" name="SpringingMinimumFixedCostCoverageRatioCovenantPercentageOfCommittedFacility" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_LineOfCreditFacilityEarlyTerminationFee" name="LineOfCreditFacilityEarlyTerminationFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_DebtMaturityMonthAndYear" name="DebtMaturityMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_EffectiveIncomeTaxRateReconciliationStateReturnToProvision" name="EffectiveIncomeTaxRateReconciliationStateReturnToProvision" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_DeferredTaxAssetsInventoryAccrualsAndReservesCurrent" name="DeferredTaxAssetsInventoryAccrualsAndReservesCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_DeferredTaxAssetDeferredIncomeNoncurrent" name="DeferredTaxAssetDeferredIncomeNoncurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_DeferredTaxAssetsLeaseLiability" name="DeferredTaxAssetsLeaseLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_TaxYear2022To2040Member" name="TaxYear2022To2040Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_TaxNotSubjectToTimeRestrictionMember" name="TaxNotSubjectToTimeRestrictionMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_PeriodOfCumulativePretaxIncome" name="PeriodOfCumulativePretaxIncome" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_PercentageOfAdjustedTaxableIncomeToOffset" name="PercentageOfAdjustedTaxableIncomeToOffset" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_PeriodOfTimeStateIncomeTaxReturnsSubjectToExamination" name="PeriodOfTimeStateIncomeTaxReturnsSubjectToExamination" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_TwoThousandElevenAndTwoThousandTwelveMember" name="TwoThousandElevenAndTwoThousandTwelveMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_AtMarketIssuanceSalesAgreementMember" name="AtMarketIssuanceSalesAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_RightsAgreementExtendedTerm" name="RightsAgreementExtendedTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" 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="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="oesx_EmployeeStockPurchasePlanClosingMarketPriceOfSharesPurchased" name="EmployeeStockPurchasePlanClosingMarketPriceOfSharesPurchased" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_SummaryOfIssuanceOfStockUnderEmployeeStockOwnershipPlanAbstract" name="SummaryOfIssuanceOfStockUnderEmployeeStockOwnershipPlanAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_ProceedsFromIssuanceOfDebtOrSaleOfEquitySecurities" name="ProceedsFromIssuanceOfDebtOrSaleOfEquitySecurities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_IncreaseInNumberOfCommonStockSharesAvailableForIssuance" name="IncreaseInNumberOfCommonStockSharesAvailableForIssuance" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="oesx_TwoThousandSixteenOmnibusIncentivePlanMember" name="TwoThousandSixteenOmnibusIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_CostOfProductsMember" name="CostOfProductsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_CostOfServicesMember" name="CostOfServicesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_RangeTwoMember" name="RangeTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_RangeThreeMember" name="RangeThreeMember" type="dtr-types: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_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueThereafter" name="EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostNotYetRecognizedDueThereafter" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_ServicesGroupMember" name="ServicesGroupMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_DistributionServicesMember" name="DistributionServicesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_USMarketsMember" name="USMarketsMember" type="dtr-types: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_CostOfProductServiceMember" name="CostOfProductServiceMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_BusinessCombinationEstimateOfEarnOutLiability" name="BusinessCombinationEstimateOfEarnOutLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_BusinessCombinationWorkingCapitalAdjustmentReceived" name="BusinessCombinationWorkingCapitalAdjustmentReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_StayLiteLightingIncMember" name="StayLiteLightingIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsRevenueEarnedButNotBilled" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsRevenueEarnedButNotBilled" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpensesAndOther" name="BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpensesAndOther" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
</xsd:schema>
