<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0304</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2011-02-16</periodOfReport>

    <dateOfOriginalSubmission>2011-02-18</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001021635</issuerCik>
        <issuerName>OGE ENERGY CORP.</issuerName>
        <issuerTradingSymbol>OGE</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001503287</rptOwnerCik>
            <rptOwnerName>Bullard William J.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>P.O. BOX 321</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>OKLAHOMA CITY</rptOwnerCity>
            <rptOwnerState>OK</rptOwnerState>
            <rptOwnerZipCode>73101</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Gen.Counsel,Okla.Gas &amp; Elect.</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock-$.01 par value per share</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>826.734</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Retirement Savings held by spouse</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The original filing included a clerical error and incorrectly omitted shares held by the reporting person's spouse in the Retirement Savings Plan. The information herein is based on a Retirement Savings Plan Statement dated February 16, 2011. The Retirement Savings Plan Statement indicated the number of units in the Common Stock Fund of the Retirement Savings Plan credited to the account of the reporting person's spouse at February 16, 2011 and includes shares credited during 2011 that were exempt from reporting pursuant to Rule 16A-3(f)(1)(i)(B). The number of shares of common stock owned at February 16, 2011, was determined by dividing the dollar value of such units by the closing sale price of the common stock on February 16, 2011.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Patricia D. Horn</signatureName>
        <signatureDate>2012-02-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
