A mouthpiece for controlled delivery of a breathing gas including oxygen, preferably an increased ratio of oxygen with respect to ambient air, to the respiratory tracts of a user which includes a valve having two modes: a closed default mode, which substantially impedes any flow of the breathing gas through the valve； and an active open mode, which allows for a substantial flow of the breathing gas through the valve, wherein the open mode can be activated by the user manipulating the valve with his/her mouth. Such a valve may be used in a device for delivery of breathing gas and in an apparatus for controlled delivery of the breathing gas to the respiratory tracts of a user. A method for controlled delivery of the breathing gas to the respiratory tracts of a user may include operating the valve.
