<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2022-08-04T22:47:58.2686387+00:00 -->
<!-- Version            : 5.0.1.747 -->
<!-- Package ID         : 5a105cedf43846008cf4b328c272bd90 -->
<!-- Copyright (c) 2022 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="oesx_ProceedsFromSaleOfUnbilledContractsReceivable" name="ProceedsFromSaleOfUnbilledContractsReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_GainLossOnSaleOfUnbilledContractRevenue" name="GainLossOnSaleOfUnbilledContractRevenue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="oesx_ChangeInContractWithCustomerLiability" name="ChangeInContractWithCustomerLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="oesx_PayrollTaxCreditCurrent" name="PayrollTaxCreditCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="oesx_EquipmentLeasedToCustomersMember" name="EquipmentLeasedToCustomersMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_AccruedProjectCostsCurrent" name="AccruedProjectCostsCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="oesx_ContractWithCustomerCreditLiabilityCurrent" name="ContractWithCustomerCreditLiabilityCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="oesx_LimitedProductWarrantyTerm" name="LimitedProductWarrantyTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_ProductWarrantyAccrualPaymentsNetOfVendorReimbursements" name="ProductWarrantyAccrualPaymentsNetOfVendorReimbursements" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="oesx_CreditAgreementMember" name="CreditAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_PriorCreditAgreementMember" name="PriorCreditAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_BankOfAmericaNAMember" name="BankOfAmericaNAMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="oesx_WesternAllianceBankMember" name="WesternAllianceBankMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <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_DebtMaturityMonthAndYear" name="DebtMaturityMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_RightsAgreementTerminateAndExpireDate" name="RightsAgreementTerminateAndExpireDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_RightsAgreementExtendedTerm" name="RightsAgreementExtendedTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <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_EmployeeStockPurchasePlanClosingMarketPriceOfSharesPurchased" name="EmployeeStockPurchasePlanClosingMarketPriceOfSharesPurchased" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <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_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedOrExercisedInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedOrExercisedInPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOrExercisedNumberOfShares" name="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOrExercisedNumberOfShares" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="oesx_DeferredCompensationArrangementWithIndividualCompensationExpensePeriodOfRecognition" name="DeferredCompensationArrangementWithIndividualCompensationExpensePeriodOfRecognition" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <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_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>
