Okla. Stat. tit. 19 § 645
The board of trustees, the city council, or the board of city commissioners of any incorporated town or city having a population of less than five thousand (5,000) inhabitants according to the last Federal Decennial Census, shall have the authority to designate the county treasurer as the official treasurer of such incorporated town or city having a population of less than five thousand (5,000) inhabitants according to the last Federal Decennial Census, who shall serve in such capacity without additional compensation, provided, however, that the board of trustees, the city council, or the board of city commissioners shall pay into the general revenue fund of the county, upon a claim filed by the county clerk an amount which the board of trustees, the city council, or the board of city commissioners and the board of county commissioners estimate will reasonably reimburse the county for supplies which the county treasurer might need to serve as treasurer of any incorporated town or city in the state. The designation of the county treasurer as treasurer of the city or town shall be by ordinance and the voters of the city or town shall not elect a treasurer while the ordinance remains in effect. When so designated, the county treasurer shall exercise all the powers and perform all of the duties of the office of treasurer of the city or town and his official bond as county treasurer shall stand for any and all moneys and securities belonging to the city or town which come into his hands.
Okla. Stat. tit. 19, § 645