Mandibular advancement devices are used for the therapy of obstructive sleep apnea. We investigated whether the therapeutically necessary CPAP value is a useful predictor of the efficacy of oral devices. We retrospectively compared polysomnography data from 67 patients, 8 women and 59 men, (age mean/standard deviation: 54.2 ± 12.3 years; BMI: 28.6 ± 4.3 kg/m²; apnea-hypopnea index: 21.9 ± 12.3/h). All patients were initially treated with CPAP. Due to problems with the CPAP mask, they switched to a mandibular advancement device. Under the use of CPAP, the apnea-hypopnea index decreased from 21.9 ± 12.3 to 3.4 ± 4.6/h, with the mandibular advancement device to 9.7 ± 11.6/h. The apnea-hypopnea index while using an oral device was remarkably different when the therapeutically necessary CPAP value was taken into consideration: in the patient group with a pressure of 4 mbar, it was 6.1 ± 8.7 and in the group with values > 8 mbar, it was 23 ± 9.6/h. Thus, the efficacy was signficantly different for CPAP values above 8 mbar. The therapeutically necessary CPAP value is a valuable predictor for the efficacy of mandibular advancement devices.