Objective: This retrospective study aimed to clarify the characteristics of positional obstructive sleep apnea (P-OSA) patients and to examine the relationship between sleep position and the efficacy of oral appliance (OA) therapy.Methods: Seventy-three patients who underwent polysomnography before and after OA therapy were divided into P-OSA or non-positional OSA (NP-OSA) and examined.Results: Forty-seven patients had P-OSA and 26 had NP-OSA. P-OSA was characterized by a relatively high ratio of supine to lateral apnea hypopnea index (AHI) because the lateral AHI of P-OSA was significantly lower than that of NP-OSA. OA therapy was more effective for P-OSA than NP-OSA. Younger age and the presence of P-OSA are useful as predictors of OA therapy efficacy.Discussion: Even for relatively severe OSA, in the case of younger age and/or P-OSA, the application of simple, cost-effective OA therapy might be recommended over other treatment modalities.