Obstructive sleep apnea (OSA) is a frequent illness related to sleep and breathing with many symptoms and complications. The primary choices of treatment are continuous positive airway pressure (CPAP) and removable appliance for protruding the mandible when sleeping (MAD). Treatment with CPAP is superior to treatment with MAD but many patients don't tolerate CPAP why they seek treatment with MAD. In Denmark, treatment with MAD is not free of charge unlike treatment with CPAP and MAD is therefore not offered systematically to the patients. The same applies to adjustment, control and effect of the MAD treatment. MAD treatment has a positive effect on OSA in general but the effect varies due to differences in selection of patients and lack of specifying who performs and evaluates MAD treatment, the effect on sleep and possible side effects in muscles and the temporomandibular joint as well as the occlusion during treatment. It is therefore of great importance to investigate the effect of MAD treatment as well as short- and long-term side effects. The aim is therefore to compare the treatment effect of MAD and CPAP and how significant the investigation of the airways and the face is for the treatment effect and possible side effects. The project is a 4-year investigation and 70 adult patients diagnosed with moderate OSA are included. The results from the investigation will be of great importance for the choice of treatment and prognosis for the individual patient and socio-economic since the treatment of OSA could be tailored the individual patient and the treatment effect optimised.