In an oxygen concentrator which is intended for aircraft use, a booster compressor (16) is used to increase the pressure of the product gas from the concentrator (15) in order to increase the amount of the gas which can be stored in a plenum (17). The booster includes two moving pistons (47,49) which are rigidly linked together and a series of check valves (44,54,57,59) which control the flow of gases through the compressor (16). One of the pistons (49) is driven by air from the rotary valve (31) in the concentrator (15), and the other piston (47) compresses the product gas for delivery to the plenum (17). A small sample of concentrator product gas is monitored by an oxygen sensor (66) for oxygen concentration. Once the oxygen concentration has reached an acceptable level the booster compressor (16) fills the plenum (17) with product gas.; Thereafter, if oxygen concentration of product gas delivered to the crew from the concentrator (15) falls below the concentration which is required at a particular altitude, the product gas stored in the plenum (17) is delivered to the crew. The oxygen sensor monitors the concentrator output product gas to the breathing regulator when the stored plenum gas is not being used.