Okla. Admin. Code § 715:10-13-5
The statutes provide that any member whose regular annual compensation was not determined as provided by law may pay the contribution on such amount not included in the member's regular annual compensation and receive credit for such amount in the calculation of the member's benefit. The member must also pay any employer contributions due TRS which are not paid by the employer. The intent of this provision and the provisions outlined above is to enable members to receive credit for their total contract salary up to applicable limits. Therefore, members' retirement contributions should be based on the gross salary before fringe benefits are subtracted, not after. No adjustments can be made until the corrections are documented as outlined in OAC 715:10-5-6. Any loss incurred by the Retirement System as a result of the failure of a school to comply with existing statutes or TRS Rules shall be paid by the member or school to the Retirement Fund. Interest compounded annually at ten percent (10%) per annum shall be charged on all balances due Teachers' Retirement because of failure to remit the proper contribution. In cases where a compensation correction should have been identified by TRS staff but was not, interest may be adjusted to correct for the error in accordance with TRS procedures.
Okla. Admin. Code § 715:10-13-5