Okla. Stat. tit. 12 § 1755
Unless service by the sheriff or other authorized person is requested by the plaintiff, the defendant shall be served by mail. The clerk shall enclose a copy of the affidavit and the order in an envelope addressed to the defendant at the address stated in said affidavit, prepay the postage, and mail said envelope to said defendant by certified mail and request a return receipt from addressee only. The clerk shall attach to the original affidavit the receipt for the certified letter and the return card thereon or other evidence of service of said affidavit and order. If the envelope is returned undelivered and sufficient time remains for making service, the clerk shall deliver a copy of the affidavit and order to the sheriff who shall serve the defendant in the time set in Section 1756 of this title, or at the election of the plaintiff service shall be governed by the provisions of subsection C of Section 2004 of this title.
Okla. Stat. tit. 12, § 1755