Oral appliance (OA) is known to be beneficial to improve the severity of obstructive sleep apnea. However, its effect on sleep quality in patients with OSA has to be elucidated. Ten patients with OSA underwent two polysomnography (PSG) sessions, one without OA and another with OA. We found that OAattendant sleep was associated with decreased stage 1 sleep (N1), increased stage 2 sleep (N2), and decreased arousal index compared to the non-OA condition. We concluded that sleeping with the OA is beneficial to sleep quality.