The patient was a 65-year-old woman who was diagnosed with obstructive sleep apnea (OSA) one month before her first visit to our clinic, and oral appliance (OA) treatment was initiated. After using the OA for three days, she developed bilateral temporomandibular joint and masseter muscle pain and posterior open bite (POB) on the right side. Use of the OA was stopped and the pain disappeared immediately; however, the right POB remained unchanged after several days. Her medical history included bilateral clicking sounds and intermittent right molar malocclusion for several years; however, after the onset of POB, the sounds on the right side disappeared, and the malocclusion persisted. Clicking sounds on the left side were observed as a clinical finding. The panoramic radiograph revealed enlargement of the space between the right mandibular condyle and the glenoid fossa. Magnetic resonance imaging showed that the right articular disc was in the normal position, while there was an anterior disc displacement without reduction on the left side. Based on the above findings, the patient was diagnosed with molar open bite caused by the displacement of the mandibular condyle due to the reduction of the right articular disc. Consequently, by performing exercise therapy, occlusal recovery was obtained without occlusal plane reconstruction. After confirming occlusal stability, the OA was recreated to resume OSA treatment, and the amount of mandibular advancement was set at 50% of the maximum. Currently, there is no recurrence of malocclusion and progress is good. Based on the present case, when performing OA treatment for OSA, it seems clinically important to start with a small amount of mandibular advancement considering its effect on the temporomandibular joint, and if sufficient therapeutic effects cannot be obtained, then the amount of movement should be reexamined while considering side effects such as pain and malocclusion.