An oxygen trainer assembly for use in athletic activities to increase inspiratory muscular endurance includes 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. The oxygen trainer assembly further includes a first endcap insert disposable in the first endcap operative to control the level of air resistance in the assembly during inhalation and exhalation and 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.