 Patients in group 1 patients with OHS will begin NIV treatment, using a BIPAP ventilator in Sponttimed mode RF 12 and nasal masks. The parameters of the respirator will be adjusted during a nap; expiratory pressure EPAP will be at least 4 cm H2O and inspiratory pressure 10-20 cm H2O, depending on the patients tolerance. Oxygen will be added until a continuous saturation of between 93-95 is obtained, and a gas sample will be taken on waking. 