A vacuum-pressure swing absorption concentrator includes a motor driven compressor having pressure and vacuum heads that are connected to a pressure reservoir and a vacuum reservoir respectively. The pressure and vacuum reservoirs are selectively and alternately interconnected in sequence through a main valve to a pair of nitrogen filtering sieve beds. A controller operates the valve to alternately and cyclically interconnect the sieve beds to the pressure and vacuum reservoirs respectively. During each cycle, a respective bed is pressurized and enriched oxygen is produced and delivered to a tank for use by a patient. At the same time, the other bed is evacuated through the vacuum reservoir. A crossover valve delivers oxygen from a pressurized bed to an evacuated bed to facilitate purging of impurities previously collected in the evacuated bed.