Abstract: Apparatus for use with a medical oxygen concentrator for supplying oxygen enriched air to a patient and to a cylinder filler. The apparatus includes a flow control valve which is controlled by a control circuit to deliver doses of oxygen enriched air to a patient during at least a portion of the patient's inhalation and to deliver oxygen enriched air to a circuit for filling a portable oxygen cylinder during the remainder of the time. In the event of a failure of the control circuit, the flow control valve delivers a continuous flow of oxygen enriched air to the patient. In the event of a failure of the oxygen concentrator, oxygen enriched air stored in the oxygen cylinder may be delivered to the patient.