Obstructive sleep apnea is a potentially life-threatening sleep disorder that significantly compromises daytime functioning. Currently, the gold standard for treating obstructive sleep apnea is Continuous Positive Airway Pressure (CPAP). However, due to non-compliance, oral appliances have been developed as an alternative treatment. While oral appliances have been shown to alleviate symptoms of obstructive sleep apnea, no studies to date have thoroughly examined their impact on daytime functioning. This study will compare the effectiveness of oral appliances to CPAP on measures of subjective and objective daytime sleepiness, objective cognitive functioning, driving performance, quality of life and mood. A total of 40 patients (20 treated with CPAP and 20 treated with an oral appliance) will be recruited to participate. Eligible participants will wear an actigraphy monitor during a one-week period to evaluate nightly total sleep time. Following the last night of monitoring, participants will spend one day in the laboratory undergoing a series of napping tests to evaluate degree of sleepiness. Additionally they will perform tests of attention and driving performance and complete questionnaires evaluating mood and quality of life. Between-group comparisons will be performed to compare the groups across the objective and subjective variables. This study has the potential to significantly contribute to the sleep medicine field by expanding the current understanding of oral appliances and CPAP with regards to daytime functioning and quality of life.