CPAP is the most effective treatment of obstructive sleep apnea. Oro-nasal masks may be used in case of mouth leaks but these are associated with higher positive pressure needs and lower compliance to treatment. The present investigation evaluates if CPAP compliance would increase when an oral appliance is used in combination with a nasal mask compared to the use of an oro-nasal mask. Eligible patients are those demonstrating a low compliance when using an oro-nasal mask during CPAP therapy. Patients will be treated with automatic CPAP with one of the above-detailed interfaces for 4 weeks and data will be extracted from the machine report in each condition.