Okla. Admin. Code § 715:10-5-1
Members may purchase credit for years of employment from July 1, 1943, to date of membership in the public schools of Oklahoma on which contributions were not remitted. One (1) full year (twelve calendar months) as a contributing member of TRS must be completed before a member may make such purchases. All purchased service must meet Teachers' Retirement System minimum requirements for eligibility, in effect at the time of purchase, and be properly documented before purchase is allowed. Payment shall not be allowed for any employment during a school year that was less than one-half (1/2) time, and no credit is allowed for periods of employment when a member participated in an alternate retirement plan as provided for by the Alternate Retirement Plan for Comprehensive Universities Act. All requests for billings to make payment for past service must be made while an active contributing member of the Teachers' Retirement System or within sixty (60) days of termination of employment in the public schools of Oklahoma. Payment for service credit must be completed 90 days prior to the effective date of retirement and a billing request cannot be initiated by any person after the death of the member. (See OAC 715:10-5-4 for cost and method of payment).
Okla. Admin. Code § 715:10-5-1