Mandibular advancement devices, more simply known as Oral Appliances (OAs), represent one of the current treatments for Obstructive Sleep Apnea Hypopnea Syndrome (OSAHS). The primary objective of the study was to measure their efficacy in the severe OSAHS population. The secondary objectives were to determine predictive factors for the OA efficacy and to determine the tolerance and frequency of treatment discontinuation.OA efficacy outcomes based on Apnea-Hypopnea Index (AHI) reduction, complications, and discontinuation rates were collected from 271 prospectively followed severe OSAHS patients treated with OAs.OA treatment reduced the AHI by a mean 21.5 respiratory events/h, P50% and 58.5% of patients exhibited AHI less than 15/h. The effectiveness of OA on AHI did not vary significantly over time (P=0.0936). OA was discontinued by 9.6% of patients, mainly due to its ineffectiveness, and more rarely because of intolerance.Mandibular advancement orthosis is an effective and well-tolerated treatment for severe OSAHS. In 70.7% of the cases studied, the oral appliance reduced AHI by at least 50%, and it should be more systematically proposed in the event of failure or refusal of CPAP treatment.