A product pump for pumping separated product gas from an oxygen concentrator which produces oxygen-rich product gas by utilizing a vacuum swing adsorption (VSA) process. The oxygen concentrator comprises a cartridge containing an adsorbent material for separating the product gas from ambient air, a vacuum pump which provides a vacuum source, a valve for selectively connecting the vacuum pump to the cartridge and product pump, and a reservoir. The product pump comprises a pumping chamber having a product gas inlet and a product gas outlet, a piston movable in the chamber, a vacuum inlet in the chamber on an opposite side of the piston from the product gas inlet and outlet for providing a vacuum to cause the piston to draw product gas through the product gas inlet into the chamber, and a spring for providing a spring force to force the product gas out the product gas outlet.