Okla. Stat. tit. 36, § 6092

No provision in an automobile liability policy or endorsement for such coverage effective in this state issued by an insurer on and after the effective date of this act which grants the insurer the right of subrogation for payment of benefits under the expenses for the medical services coverage portion of the policy, to a named insured under the policy, or to any relative of the named insured who is a member of the named insured's household shall be valid and enforceable; provided, that such policy or endorsement may provide for said insurer's rights of subrogation and set-off upon such payments to any person who is not a named insured under the policy or a relative of the named insured who is a member of the named insured's household.