Okla. Stat. tit. 26 § 5-125
Service shall be made by the sheriff of the county of residence of the contestee as to all offices, except that of sheriff, in which case the same shall be served by the county clerk of the appropriate county, and the certificate of returns of such sheriff or county clerk, showing the inability to make such service within the aforementioned time, shall be deemed sufficient proof of the absence of the contestee, or the inability to serve such petition and notice upon him, and to justify the constructive service herein provided. When personal service is impossible, within said time, it is hereby made the duty of said petitioner to serve said true copies upon the secretary of the appropriate election board. Provided that for the purpose of such constructive service, the Secretary of the State Election Board and the secretaries of the county election boards are hereby made and constituted the service agents for all candidates who file declarations of candidacy with them. By filing his declaration of candidacy, each candidate shall thereby be conclusively presumed to have accepted the terms and provisions hereof and specifically the aforesaid constructive service. When constructive service becomes necessary, said constructive service shall be made at the date, time and place of said hearing, after proof of inability to personally serve the contestee has been returned to the petitioner.
Okla. Stat. tit. 26, § 5-125