Okla. Admin. Code § 310:2-21-4
Any instruments initiating an administrative proceeding must be served on every named Respondent by either personal service, certified mail, return receipt requested, restricted delivery, or issuing a report by hand-delivery. If service is being sent by certified mail, return receipt requested, and the intended Respondent refuses to sign the return receipt or otherwise does not sign or is unavailable to sign and accept service through the certified mail at the address identified on Department records, then Respondent is deemed to have been served, If service is by personal service, the person serving the instrument initiating an administrative proceeding shall file proof of service with the Hearing Clerk within twenty (20) days of service or before the date of the first hearing, whichever is sooner. If an inspection is performed, the report and/or the notice to correct violations issued by the inspectors/sanitarians to the license holder or to the person in charge, requesting a signed acknowledgement of receipt of the report or notice, shall constitute service of the report and/or notice. Acknowledgement in writing by the Respondent or appearing at the hearing without objection to service is equivalent to service.
Okla. Admin. Code § 310:2-21-4