A mask for delivery of oxygen to a patient, the mask comprising: a body having a peripheral portion, when in use to sit comfortably on a patient's face, a central portion, and bridge portions extending between the central portion and the peripheral portion and integral therewith, the central portion having an inner surface and an outer surface, the inner surface to be oriented towards the patient's face when the mask is in position and contoured so as to sit at a location spaced over the patient's nose and mouth, the inner surface of the central portion provided with a wall circumscribing a base, the wall and base being of generally concave configuration and circumscribing a centrally positioned oxygen delivery aperture extending through the central portion between the inner surface and the outer surface, the wall and base configured so as to act as an oxygen diffuser to direct the flow of oxygen generally towards the patient's nose and mouth when the mask is in use; a fastener to engage the mask to the patient so as to maintain contact between the patient's face and said peripheral portion when the mask is in use; means associated with the aperture of the central portion to releasably receive and secure in position an oxygen delivery tube; said diffuser being positioned within the mask; and abaffle (52, 64) associated with said diffuser in the path of the flow of oxygen; wherein the diffuser wall and base are of cup-shaped appearance, the wall ending in an edge of triangular peripheral contour; wherein the diffuser and baffle are shaped so as to generate a plume of turbulent flow of oxygen-enriched gas which surrounds the user's nose any preferably also his mouth, when a gas discharge rate of 1-15 liters/minute is provided; and wherein the mask body comprises a substantially open structure having openings which together comprise between 30 and 80% of the total surface area of the mask body. a body having a peripheral portion, when in use to sit comfortably on a patient's face, a central portion, and bridge portions extending between the central portion and the peripheral portion and integral therewith, the central portion having an inner surface and an outer surface, the inner surface to be oriented towards the patient's face when the mask is in position and contoured so as to sit at a location spaced over the patient's nose and mouth, the inner surface of the central portion provided with a wall circumscribing a base, the wall and base being of generally concave configuration and circumscribing a centrally positioned oxygen delivery aperture extending through the central portion between the inner surface and the outer surface, the wall and base configured so as to act as an oxygen diffuser to direct the flow of oxygen generally towards the patient's nose and mouth when the mask is in use; a fastener to engage the mask to the patient so as to maintain contact between the patient's face and said peripheral portion when the mask is in use; means associated with the aperture of the central portion to releasably receive and secure in position an oxygen delivery tube; said diffuser being positioned within the mask; and abaffle (52, 64) associated with said diffuser in the path of the flow of oxygen; wherein the diffuser wall and base are of cup-shaped appearance, the wall ending in an edge of triangular peripheral contour; wherein the diffuser and baffle are shaped so as to generate a plume of turbulent flow of oxygen-enriched gas which surrounds the user's nose any preferably also his mouth, when a gas discharge rate of 1-15 liters/minute is provided; and wherein the mask body comprises a substantially open structure having openings which together comprise between 30 and 80% of the total surface area of the mask body.