Oklahoma Naturalization and Citizenship
Immigrants seeking to become citizens often applied at a U.S. District Court or a district court at a county courthouse. You can obtain copies of declarations, certificates, and other records by contacting the clerk's office in the county. Contact details for Oklahoma county courts can be found on http://www.genealogy.com/00000265.html. The Family History Library has microfilm copies from some counties.
Post 1906 Records
For naturalization records after September 1906, contact the National Archives—Southwest Region or use the Genealogy Program at www.uscis.gov. The Southwest Region branch at Fort Worth has records of the U.S. District Court for the Indian Territory: 1889-1907; Oklahoma Eastern District: 1907-1988; Oklahoma Western District: 1907-1956; and Oklahoma Northern District: 1925-1969.
Oklahoma Research Outline. Salt Lake City, Utah: Intellectual Reserve, Inc., Family History Department, 1998, 2001.