Background : Long-term oxygen therapy is prescribed for patients with severe COPD. The aim of oxygen therapy is to administer oxygen at a concentration level higher than that of ambient air, in order to treat or prevent the symptoms and manifestations of arterial hypoxemia; this therapy can be applied either in an acute situation or as a long-term treatment, in cases of stabilized severe chronic hypoxemia. Currently, pneumologists do not have the possibility, between 2 consultations spaced several months apart, to measure the patient's compliance with the treatment (adherence to oxygen therapy) nor the respect of the prescribed dosage (O2 flow and duration). In addition to the patient's adherence to the treatment, the specialist does not have the possibility to assess the patient's physical activity (walking ...). This observational study is a e-health, prospective, multicenter study conducted in France under the control of pneumologists. The main objective of this study is to evaluate the adherence to oxygen therapy in real life, and its evolution, in COPD patients justifying a long term oxygen therapy in stable state.