Okla. Stat. tit. 60 § 175.10
A corporate trustee may deposit with itself trust funds in checking and savings accounts, savings certificates, certificates of deposit, and any other type of demand or time deposit, provided it maintains under control of its trust department, if it has a trust department separate from its banking department, as security for such deposit a separate fund consisting of securities legal for trust investments which have at all times during the deposit a total market value exceeding the amount of the deposit. No such security shall be required to the extent said deposit is guaranteed by or under state or federal law.
The separate fund of securities shall be marked as such. Withdrawals from or additions to it may be made from time to time, as long as the required value is maintained. The income of such securities shall belong to the corporate trustee.
Okla. Stat. tit. 60, § 175.10