Okla. Stat. tit. 68, § 2385.24

Beginning July 1, 1989, all state and county retirement systems whose benefits were exempt from state income taxation prior to January 1, 1989, and all municipal retirement systems whose benefits were subject to state income taxation prior to January 1, 1989, shall begin to withhold monies, as required by Sections 2385.1 et seq. of this title, for state income tax purposes from the benefits paid to each member of the retirement systems, unless the member requests the retirement system not to withhold monies for state income tax purposes.