Okla. Stat. tit. 19, § 294

The register of deeds of any county of this state shall, in addition to the duties now prescribed by law, make and keep indexes in the same manner as is now provided by Sections 1733 and 1737 of Snyder's Compiled Laws, 1909, of all records, files, books, instruments, or the certified copy thereof, pertaining to the title to lands in such county, which were of record in the office of United States Court, or any office maintained by the United States prior to November 16, 1907, for filing and recording deeds, mortgages, liens and other instruments which have been, or may hereafter be, received by such register of deeds.