Okla. Admin. Code § 715:10-5-2
A member of Teachers' Retirement System who was employed as a substitute teacher may purchase credit for such employment provided they were employed for 120 days in one school year. Such service shall be considered the equivalent of one (1) year of service. Members claiming credit for such service, and who have such service documented through Teacher Personnel, State Department of Education, must pay the actuarial cost of such service as defined by OAC 715:10-5-4. The maximum creditable service granted for substitute teaching shall be five (5) years. Under no condition may substitute teaching service from one school year be combined with substitute teaching service or any other employment from another school year to obtain 120 days of substitute teaching credit. The payment for such service credit may be made in one lump sum or in equal monthly installments up to sixty (60) months, as provided in OAC 715:10-5-4(9).
Okla. Admin. Code § 715:10-5-2