Abstract: The present invention is an oxygen sensor, having the form of an electrochemical cell, of the galvanometric type. Oxygen from the outside environment enters the cell through a gas-permeable liquid-impermeable membrane, and supports an electrochemical reaction which generates an electric current. The amount of current produced is proportional to the amount of oxygen entering the cell. The cathode of the cell is a flat, perforated gold disk, and the connection between the cathode and an external terminal is made with a spring which is compressed between the terminal and the cathode. In order to seal the cell against leakage of liquid, the cathode is enclosed within an O-ring, and the membrane is placed directly over the cathode and O-ring combination. In an alternative embodiment, suitable for use in high-pressure environments, the cell is provided with an expansion diaphragm.