Okla. Stat. tit. 18 § 907
The business and affairs of a corporation shall be conducted by a board of directors. The number of directors shall be a multiple of three with a minimum of fifteen and a maximum of twenty-one. Two-thirds of the directors shall be elected by the members, and one-third shall be elected by the stockholders. One-third of the original board shall be elected for a term of one (1) year, one-third for a term of two (2) years, and one-third for a term of three (3) years; all directors subsequently elected shall serve for a term of three (3) years. Any vacancy in the office of a director elected by the members shall be filled by the directors elected by the members, and any vacancy in the office of a director elected by the stockholders shall be filled by the directors elected by the stockholders.
Okla. Stat. tit. 18, § 907