Objectives: The present study was designed to investigate the changes in occlusion caused by dental side effects of oral appliance therapy in obstructive sleep apnea syndrome compared with continuous positive airway pressure (CPAP) therapy. Methods: Twenty cases (n=20) over 20 years of age and diagnosed with obstructive sleep apnea syndrome (OSAS) (apnea–hypopnea index AHI >5) based on polysomnography were selected and randomized for either Mandibular advancement splint (MAS) (n10) or a tongue stabilizing device (TSD) (n10) therapy. Tested and compared dental side effects in both groups of patients treated with oral appliances versus control group (n10) composed of OSAS patients elected to be treated with continuous positive airway pressure (CPAP). From all patients, dental impressions were obtained an digitally scanned at baseline and after 6 months of treatment to digitally determine variables related to dental changes and occlusion. Mean and standard deviation were used for summarizing data. Student's t test was used for testing significant results between two sample means; paired t test was used for testing significant results between mean values for the same individuals before and after intervention. Significant results were considered if p<0.05, high significant results were considered if p<0.01. Results: Oral appliance therapy showed small but significant dental changes compared with CPAP. In the MAS group, overbite and overjet decreased 1.2 (±1.1) mm and 1.5 (±1.5) mm, respectively. Furthermore, we found a significantly larger antero–posterior change in the occlusion (−1.3±1.5 mm) compared to the CPAP group (−0.1±0.6 mm). In addition, in all groups, a significant decrease in number of occlusal contact points in the premolar region was found. Conclusion: we found that using an oral appliance therapy and CPAP for 6 months may result in dental changes in OSAS patients, particularly an MAS appliance, where a TSD appliance showed less impact on occlusion.