In his discretion, an appointing officer may fill any position in the competitive service either by competitive appointment from a civil service register or by noncompetitive selection of a present or former Federal employee, in accordance with the Civil Service Regulations. He shall exercise his discretion in all personnel actions solely on the basis of merit and fitness and without regard to political or religious affiliations, marital status, or race.

OPM, whenever it determines it to be necessary, shall prescribe regulations governing the release of employees (both within the competitive service and the excepted service) by any agency in the executive branch of the Government for employment in any other agency, and governing the establishment, granting, and exercise of rights to reemployment in the agencies from which employees are released.

(a) No person shall be admitted to competitive examination unless such person is a citizen or national of the United States.

(b) No person shall be given any appointment in the competitive service unless such person is a citizen or national of the United States.

(c) OPM may, as an exception to this rule and to the extent permitted by law, authorize the appointment of aliens to positions in the competitive service when necessary to promote the efficiency of the service in specific cases or for temporary appointments.