An oxygen trainer assembly for use in athletic activities to increase inspiratory muscular endurance, the assembly comprising: a mouthpiece having a mouthpiece opening； a circulation chamber housing having a circulation chamber in fluid communication with the mouthpiece opening； a first endcap having a first endcap aperture and a second endcap having a second endcap opening in fluid communication with the circulation chamber housing； a first endcap insert disposable in the first endcap operative to control the level of air resistance in the assembly during inhalation and exhalation； a one-way valve disposable in the second endcap sized and configured to allow the flow of air away from the circulation chamber during exhalation.
