Chronic obstructive pulmonary diseases (COPD) coexisting with obstructive sleep apnea is called overlap syndrome (OS). Patients with OS seem to have worse prognosis than patients with only one disorder. Noninvasive positive pressure ventilation is an efficient treatment in obstructive sleep apnea, but the effectiveness in improving outcomes of OS patients is still not fully investigated. The aim of this non-randomized concurrent control trial is to evaluate noninvasive positive pressure ventilation's effectiveness in OS patients.