A respirator with a delivery device, which draws in ambient air via an inlet line and pumps it into an outlet line, with a discharge opening, which branches off from the outlet line and via which a partial flow of the gas being delivered, which can be set with a discharge valve, escapes. Oxygen is added with the smallest possible loss of gas and with good constancy of the concentration by: providing the inlet line with a buffer volume, via which the ambient air drawn in can flow; providing a return line which introduces the partial flow into the buffer volume is present between the discharge opening and the buffer volume; and providing the oxygen source connected to the inlet line and/or to the buffer volume.