An oxygen trainer assembly comprising: a mouthpiece having a mouthpiece opening; a circulation chamber housing having a circulation chamber in fluid communication with the mouthpiece opening, the circulation chamber housing having a first tab extending from the first chamber housing end and a second tab extending from the second chamber housing end, the first chamber housing end having a first circulation aperture and the second chamber housing end having a second circulation aperture; a first endcap mountable to the first tab, the first endcap having a first endcap aperture in fluid communication with the first circulation aperture; a second endcap mountable to the second tab, the second endcap having a second endcap opening in fluid communication with the second circulation aperture; a plurality of first endcap inserts disposable in the first endcap, each of the first endcap inserts having a first endcap insert opening, the first endcap insert opening being aligned between and in fluid communication with the first endcap aperture and the first circulation aperture, the first endcap insert opening sized and configured to control the level of air resistance into the circulation chamber during inhalation and out of the circulation chamber during exhalation, wherein the oxygen trainer assembly may use one of the plurality of first endcap inserts, each first endcap insert being sized and configured to control the level of air resistance into and out of the circulation chamber housing during the inhalation and exhalation of oxygen.; and a one-way valve disposable in the second endcap, the one-way valve being sized and configured to allow the flow of air from the second circulation aperture and away from the circulation chamber during exhalation and to block the flow of air from the second circulation aperture and into the circulation chamber during inhalation. a mouthpiece having a mouthpiece opening; a circulation chamber housing having a circulation chamber in fluid communication with the mouthpiece opening, the circulation chamber housing having a first tab extending from the first chamber housing end and a second tab extending from the second chamber housing end, the first chamber housing end having a first circulation aperture and the second chamber housing end having a second circulation aperture; a first endcap mountable to the first tab, the first endcap having a first endcap aperture in fluid communication with the first circulation aperture; a second endcap mountable to the second tab, the second endcap having a second endcap opening in fluid communication with the second circulation aperture; a plurality of first endcap inserts disposable in the first endcap, each of the first endcap inserts having a first endcap insert opening, the first endcap insert opening being aligned between and in fluid communication with the first endcap aperture and the first circulation aperture, the first endcap insert opening sized and configured to control the level of air resistance into the circulation chamber during inhalation and out of the circulation chamber during exhalation, wherein the oxygen trainer assembly may use one of the plurality of first endcap inserts, each first endcap insert being sized and configured to control the level of air resistance into and out of the circulation chamber housing during the inhalation and exhalation of oxygen.; and a one-way valve disposable in the second endcap, the one-way valve being sized and configured to allow the flow of air from the second circulation aperture and away from the circulation chamber during exhalation and to block the flow of air from the second circulation aperture and into the circulation chamber during inhalation.