An oxygen therapy face mask has a face engaging portion and a valve assembly connected thereto. The valve assembly has a first passage communicating with the interior of the face engaging portion from which gas can flow into the face engaging portion and into a patient's mouth and nose and which can also receive gas from a patient's mouth and nose and the interior of the face engaging portion. A first one-way valve is associated with the first passage and a second passage connectable to an oxygen reservoir bag and to an external source of pressurized oxygen. The first one-way valve is operable to permit flow of gas from the second passage to the first passage and to prevent flow of gas from the first passage to the second passage. A second one-way valve is associated with the first passage and is operable to permit flow of gas from the first passage to the external atmosphere and to prevent flow of gas from the external atmosphere to the first passage. A third one-way valve is associated with the first passage and is operable to permit flow of gas from the external atmosphere to the first passage and to prevent flow of gas from the first passage to the external atmosphere. A patient can inhale oxygen which passes from the second passage through the first one-way valve and can also inhale air from the external atmosphere through the third one-way valve if sufficient oxygen is not available in the second passage.