Objective To evaluate the effectiveness of oral appliance (OA) vs. continuous positive airway pressure (CPAP) in treating patients with mild to moderate obstructive sleep apnea-hypopnea syndrome (OSAHS). Methods The following databases including PubMed, EMbase, The Cochrane Library, CBM, VIP, WanFang data and CNKI were searched from inception to November 30, 2012 to collect the randomized controlled trials (RCTs) on OA vs. CPAP in treating OSAHS. The relevant conference proceedings were also retrieved without limitation of type and publication time. In accordance with the inclusion and exclusion criteria, two reviewers independently screened studies, extracted data, and evaluated quality. And then meta-analysis was performed using RevMan 5.1 software. Besides, the level of evidence was graded using GRADEpro 3.6 software. Results A total of 7 RCTs were included. The results of meta-analysis showed that: a) compared with OA, CPAP significantly reduced the degree of apnea-hypopnea index (AHI) (WMD=9.13, 95%CI 8.77 to 9.50, Plt;0.000 01); and b) there was no significant difference in the Epworth sleeping scale (ESS) between OA and CPAP (WMD=0.00, 95%CI −0.12 to 0.12, P=0.97). Conclusion Compared with OA, CPAP takes remarkable effects in improving AHI for mild to moderate OSAHS, but it shows no significant difference in improving ESS. For the quality and quantity limitation of the included studies, this conclusion still needs to be proved by conducting more high quality RCTs.