Okla. Stat. tit. 62, § 62-57.134: Investment of sinking fund monies

The State Treasurer shall invest all sinking fund monies in

direct obligations of the United States of America or in

certificates of deposits from banks in the State of Oklahoma

acceptable as depositories by the State Treasurer, when such

certificates of deposits are secured by acceptable collateral and

yield as much or more than direct obligations of the United States

of America, to mature in time to meet the principal and interest

payments on the bonds, which earnings shall be deposited in the

sinking fund.