A portable oxygen concentrator includes a pair of sieve beds having first and second ends, a compressor for delivering air to the first ends of the sieve beds, a reservoir communicating with the second ends of the sieve beds, and an air manifold attached to the first ends of the sieve beds. The air manifold includes passages therein communicating with the compressor and the first ends of the sieve beds. A set of valves is coupled to the air manifold, and a controller is coupled to the valves for selectively opening and closing the valves to alternately charge and purge the sieve beds to deliver concentrated oxygen into the reservoir. An oxygen delivery manifold communicates with the second ends of the sieve beds for delivering oxygen from the reservoir to a user. Pressure sensors may be provided in the reservoir and/or delivery line for controlling operation of the controller.