A portable oxygen concentrator includes a pressure swing absorption system defined by a relatively rigid housing adapted to generate a flow of oxygen enriched gas and a battery adapted to provide power to the pressure swing absorption system. The oxygen concentrator has a total weight of less than about 10 lbs, has a maximum flow of 100% O2 equivalent gas of about 0.9 lpm, has a total volume less than about 800 in3, and gas a battery life of at least about 8 hours. The present invention also using a liquefaction or transfill system in combination with such an oxygen concentrator.