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

    <schemaVersion>X0609</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2026-07-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001517518</issuerCik>
        <issuerName>ClearBridge Energy Midstream Opportunity Fund Inc.</issuerName>
        <issuerTradingSymbol>EMO</issuerTradingSymbol>
        <issuerForeignTradingSymbol></issuerForeignTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001889539</rptOwnerCik>
            <rptOwnerName>Corebridge Financial, Inc.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>2919 ALLEN PARKWAY, WOODSON TOWER</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77019</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>See Remarks</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000005108</rptOwnerCik>
            <rptOwnerName>AMERICAN GENERAL LIFE INSURANCE CO</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>2727-A ALLEN PARKWAY</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77019</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>See Remarks</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Series J Mandatory Redeemable Preferred Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-07-23</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>134286</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>35.23</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held through subsidiaries</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Series P Mandatory Redeemable Preferred Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>160000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held through subsidiary</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>3.56% Series N Senior Secured Notes due June 11, 2027</value>
            </securityTitle>
            <postTransactionAmounts>
                <valueOwnedFollowingTransaction>
                    <value>1492293.86</value>
                </valueOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held through subsidiary</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>3.76% Series O Senior Secured Notes due June 11, 2030</value>
            </securityTitle>
            <postTransactionAmounts>
                <valueOwnedFollowingTransaction>
                    <value>2051904.07</value>
                </valueOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held through subsidiaries</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The outstanding shares of Series J Mandatory Redeemable Preferred Stock were redeemed by the issuer at maturity at a price equal to the liquidation value of $35.00 of the redeemed securities plus accrued interest of approximately $0.23 per share.</footnote>
        <footnote id="F2">Directly held by American General Life Insurance Company (&quot;AGLIC&quot;), an indirect wholly owned subsidiary of Corebridge Financial, Inc. (&quot;CRBG&quot;).</footnote>
        <footnote id="F3">AGLIC and The United States Life Insurance Company in the City of New York, an indirect wholly owned subsidiary of CRBG, directly hold $1,380,371.82 principal amount and $671,532.25 principal amount of the reported securities, respectively.</footnote>
    </footnotes>

    <remarks>Filed pursuant to Section 30(h) of the Investment Company Act of 1940.</remarks>

    <ownerSignature>
        <signatureName>/s/ Polly Klane, Authorized Signatory of Corebridge Financial, Inc.</signatureName>
        <signatureDate>2026-07-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Mary Brodd, Authorized Signatory of American General Life Insurance Company</signatureName>
        <signatureDate>2026-07-27</signatureDate>
    </ownerSignature>
</ownershipDocument>
