Okla. Stat. tit. 21, § 62.1

Whenever any person is declared punishable for a crime by imprisonment in the penitentiary for a term not less than any specified number of years, and no limit to the duration of such imprisonment is declared, the court authorized to pronounce judgment upon such conviction may, in its discretion, sentence such offender to imprisonment during the natural life of the offender, or for any number of years not less than such as are prescribed.