14 U.S.C. § 2166

(a) In General .— The Commandant may authorize an officer in a grade above grade O–2 to remain on active duty after the date otherwise provided for the retirement of such officer in section 2154 of this title , if the officer possesses a critical skill, or specialty, or is in a career field designated pursuant to subsection (b). (b) Critical Skills, Specialty, or Career Field .— The Commandant shall designate any critical skill, specialty, or career field eligible for continuation on active duty as provided in subsection (a). (c) Duration of Continuation .— An officer continued on active duty pursuant to this section shall, if not earlier retired, be retired on the first day of the month after the month in which the officer completes 40 years of active service. (d) Policy .— The Commandant shall carry out this section by prescribing policy which shall specify the criteria to be used in designating any critical skill, specialty, or career field for purposes of subsection (b).