Abstract: A backup breathing gas supply is used with an oxygen concentrator system in which breathing gas is derived as an oxygen enriched product gas from the oxygen concentrator system and delivered to a breathing regulator. The backup breathing gas supply includes a plenum chamber for storing a quantity of breathing gas, a plenum line coupling the output of the oxygen concentrator to the plenum chamber, and a bypass line coupled to the output of the oxygen concentrator. During normal operation, breathing gas is delivered by means of the bypass line. In the event of loss of air supply to the concentrator, loss of electrical power, or concentrator breakdown, breathing gas is delivered from the plenum chamber. A three-way control valve is used to selectively couple either the plenum chamber or the bypass line to the system outlet.