An oxygen concentrator comprises an oxygen reservoir and adsorbent columns that each have an inlet, an outlet, and a bed of adsorbent material. The concentrator also comprises an air inlet, an exhaust outlet, and a vacuum pump for pumping nitrogen rich gas from one of the column inlets to the exhaust outlet. A product control pump pumps oxygen rich gas from one of the column outlets to the oxygen reservoir. A control valve controls flow in and out of the columns by selectively connecting the air inlet to one column inlet, connecting the vacuum pump to another column inlet, and connecting the product control pump to a column outlet. A motor drives the vacuum pump and control valve to produce vacuum swing adsorption (VSA) cycles in which oxygen-rich product gas is separated and accumulated in the reservoir.