Okla. Stat. tit. 18, § 381.38

Any association may classify its deposit accounts according to the character, amount or duration thereof, or regularity of additions thereto, and may pay additional or higher rates of earnings on accounts based on such classifications than is paid on regular deposit accounts, provided that any such higher rate, or bonus, to be paid on any class of accounts shall not exceed the limitations prescribed by the State Banking Commissioner or by applicable federal regulations. A mutual association may also classify its accounts according to type of account, such as full paid, single payment, installment, optional installment, bonus or other types of accounts designated by the bylaws and permitted by the Commissioner.