Okla. Stat. tit. 82 § 669
In all suits in any court for damages, condemnation or other proceedings, whether brought for or against such conservancy district, the same shall be brought in the name of the conservancy district as fixed and established by the decree of incorporation, and shall be under the direction of its board of directors and in all suits against such conservancy district service of summons shall be issued and served upon the president of the board of directors; or, if the president of said board of directors is not found in the county, then upon a member of the board of directors of said district, or if none of the aforesaid directors can be found, then by serving the secretary or treasurer and by leaving a copy thereof at the principal office or usual place of business of said district, with the person having charge thereof.
Okla. Stat. tit. 82, § 669