Okla. Stat. tit. 74, § 913.1a

A member may receive credit in the Oklahoma Public Employees Retirement System for those years of service accumulated by the member as nonclassified optional personnel as defined by paragraph 4 of Section 17-101 of Title 70 of the Oklahoma Statutes if the member is not receiving or eligible to receive retirement credit or benefits from said service in any public retirement system. To receive the service credit, the member shall pay the amount determined by the Board pursuant to Section 913.5 of this title.