Oxygen supplementation has been proven to be effective in hypoxemic COPD patients by increasing oxygenation and reducing dyspnea. In clinical practice there are three common oxygen delivery systems used: continuous oxygen flow (CF), demand oxygen delivery with liquid oxygen (DDL) and demand oxgen delivery with portable oxygen concentrator (DDC). The CF involves considerable wastage of oxygen because oxygen is supplied during in- and exhalation. Demand oxygen delivery saves oxygen and results in a comparable oxygen saturation at rest in COPD patients. However, it is unknown, if oxygen supplementation via demand oxygen delivery is also sufficient during exercise in hypoxemic COPD patients. In addition, it has to be considered that the lower weight of demand oxygen delivery system might enable patients for higher physical activity level and mobility in daily life.