Okla. Stat. tit. 19, § 215.20

At the primaries and general elections held in 1966, and each four (4) years thereafter, there shall be nominated and elected in each and every district attorney's district in the State of Oklahoma, a district attorney, who shall hold office for a term of four (4) years, beginning on the first Monday of January following his election, and until his successor is elected and qualified.