Okla. Admin. Code § 715:10-1-6
Date of membership is the date the initial contribution is made to TRS under the current membership account. Any former member of TRS who has previously withdrawn contributions and who redeposits said withdrawn contributions as permitted by law shall have his or her initial date of membership reinstated. Any person who transfers service from the Oklahoma Public Employees Retirement System in accordance with 70 O.S., Section 17-116.2(L), shall be eligible to use his or her initial entry date into the Oklahoma Public Employees Retirement System as his or her date of membership in TRS for all purposes except the member shall not be considered an ''eligible participant'' under OAC 715:10-15-27 unless the member first joined TRS prior to July 1, 1996. If a current member purchases non-contributory service for those years of qualified employment prior to the current date of membership, the official date of membership will remain the date the member's current membership account was opened. If a member purchases adjunct service that was performed prior to current date of membership, such purchased service shall be considered contributing service for purposes of vesting and membership date.
Okla. Admin. Code § 715:10-1-6