Intervention: The SPT is a sensor positioned in an elastic band attached around the body. The SPT measures the body position and vibrates when the patient lies in supine position. Oral appliance therapy (OAT) is an intra-oral prosthesis, which holds the mandible in a protrusive position. Because of this position more pharyngeal space will be available and the AHI will decrease. After randomisation the first group (n=45) will sleep for a period of 90 +/- 2 days with the SPT every night. The second group (n=45) will also sleep for a period of 90 +/- 2 days only with OAT. After this period the PSG is repeated. Long- term outcome in AHI is measured by repeating the PSG after 1 year. Main study parameters/ endpoints: Primary endpoints; reduction of PSG parameters, in particular AHI, AI, HI, DI, reduction of % of sleeping supine sleep position, without disturbance of the sleep quality.