Recently, attention to oral appliance therapy, as mandibular advancement device (MAD), has increased. Mandibular advancement device is a popular alternative treatment, particularly for mild to moderate obstructive sleep apnea (OSA). Oral appliances for treating severe OSA are recommended for patients who failed to comply with continuous positive airway pressure treatment. This clinical report presents the treatment outcome of oral appliance therapy and bilevel device in a 70-year-old man with severe OSA and also having respiratory disturbance index (RDI) of 62.7. Polysomnographic (PSG) evaluation was performed before and about 1 week after continuous use of the MAD and then with bilevel device. The results were compared, and RDI decreased 27.5 per hour with the custom MAD, whereas RDI decreased 3.1 per hour with bilevel device. Improved PSG parameters showed that bilevel device was efficient in treatment.