Okla. Stat. tit. 56, § 238.7

A final order of the Department of Human Services providing for the payment of child support which has not been set aside on appeal by the district court shall not be modified retroactively or payment of all or a portion of the past due amount waived, except by mutual agreement of the obligor and obligee or, if the child support rights have been assigned to the Department or other entity, by agreement of the Department or such entity.