A non-rebreathing oxygen mask for supplying a continuous flow of oxygen to a patient. The non-rebreathing oxygen mask includes a face mask portion for enclosing the mouth and nose of a patient, an oxygen reserve bag for receiving a supply of oxygen from an external source, and an intermediate oxygen entry port for regulating the flow of oxygen into the oxygen reserve bag from an external source. The mask further includes a closeable aperture through which an endoscope or larger sized medical instrument may be inserted, and an auxiliary tube for monitoring carbon dioxide exhalation by the patient.
