The effectiveness of a tongue-retaining device (TRD) was examined at the baseline without any TRD [TRD(-)] and then with a TRD [TRD(+)] in eight patients with obstructive sleep apnea (OSA). In terms of the 3+ % oxygen desaturation index (ODI3), lowest percutaneous oxygen saturation (SpO2), and time spent SpO2 < 90%, there were significant differences (P < 0.01-0.05) between the findings for TRD(-) and TRD(+). The ODI3 dropped below the baseline by more than 50% in five patients (63%) and dropped to fewer than 10 events/ h in six patients (75%). The results indicate that oral appliances help patients with mild to moderate OSA; however, patients with more severe OSA may also be treated effectively with a TRD.