Objective: Through this study, we aimed to evaluate the efficacy and safety of the intraoral negative air pressure device (iNAP) in patients with moderate to severe obstructive sleep apnea (OSA) in Taiwan. Design: Crossover and evaluator-blind, self-control design. Setting: Academic medical center. Patients: A total of 35 patients provided their consent to participate in this study; however, only 34 patients (30 men [87.5%] and four women [12.50%]) were eligible and randomized to the Clinical Cohort and Safety Cohort. The mean age of the 32 evaluable patients (PE cohort) was 47.4 ± 11.2 years, and their mean BMI was 26.5 ± 3.2 kg/m2. Measurements and results: The clinical response rate was 75% (24/32 patients) comparing the treatment polysomnography values to the baseline values. The mean (±standard deviation) baseline apnea-hypopnea index was 32.0 ± 11.3 events/h, which decreased significantly to 8.7 ± 9.4 events/h. Mo medical device-related adverse event or serious adverse event occurred during the study period. Conclusions: Compared with the previous oral pressure therapy device, the iNAP treated approximately three-fourths of the patients with OSA and had a superior comfort and safety profile. Thus, the iNAP device could be an alternative treatment solution for patients with moderate to severe OSA.