A method for dissolving oxygen into water comprised of diffusing oxygen in small, fine bubbles at the bottom of a water column in a closed vessel, building headspace pressure above the water column in the vessel, and circulating the oxygen from the headspace continuously till the desired oxygen dissolved concentrate of water is obtained. An apparatus for dissolving oxygen into water, comprised of an enclosed vessel, porous metallic diffusers, an agitator, a rotary hollow-shaft extending till the bottom end of the water column, a venturi slot on the hollow-shaft configured to stay above the water level, and a disc at the bottom end of the rotary hollow-shaft.