The purpose of this study is to determine the measurement techniques that can predict the effectiveness of an oral appliance (OA) for the treatment of patients with obstructive sleep apnea hypopnea syndrome (OSAHS). Split-night polysomnography and esophageal pressure (Pes) were recorded, and cephalometric tracings were superimposed for 25 OSAHS patients. The patients were classified into good and poor responders according to their apnea hypopnea index (AHI) and mean nadir Pes. When the degree of anterior displacement of the mandible was expressed by vector resolution, it was significantly different between the good and poor responders, whereas there was no significant difference in downward and total mandibular displacement between the two groups. Among the good responders evaluated on the basis of mean nadir Pes, their apnea index, hypopnea index, and mean nadir Pes were significantly different. However, AHI alone cannot predict the effectiveness of OA treatment. Good responders defined by mean nadir Pes also had short soft palates and a wide pharyngeal airway space. Conversely, no significant differences were observed in these parameters when good responders and poor responders are defined by AHI. Logistic regression analysis revealed that the degree of anterior displacement of the mandible showed a significant odds ratio of 1.97. In conclusion, evaluations based on Pes and analyses of the mandibular displacement expressed by vector resolution using a cephalometric superimposition technique can provide important clinical information in evaluation measurements and may be useful for the prediction of the efficacy of OA treatment for patients with OSAHS.