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Sponsorships or competing interests that may be relevant to content are dis-closed at the end of this article. Objective. To use drug-induced sedation endoscopy (DISE) to identify locations and patterns of residual collapse in patients with obstructive sleep apnea (OSA) with incom-plete response to oral appliance therapy (OAT). Study Design. Case series with chart review. Setting. Academic multidisciplinary sleep practice. Subjects and Methods. Thirty-five consecutively screened adult patients with OSA with continuous positive airway pressure (CPAP) intolerance and incomplete response to OAT (apnea-hypopnea index [AHI].15 or AHI.5 with persistent subjec-tive symptoms) who underwent DISE with and without the oral appliance. Data collected included demographics, body mass index, polysomnography data, and management decisions after DISE. Each DISE video pair was retrospectively scored using the VOTE classification system by the same blinded reviewer (R.J.S.). Results. All patients had multilevel airway collapse at baseline. The palate was the most common location of OAT failure. Fifteen (42.9%) had persistent collapse of the velum during DISE with OAT, and 7 (20%) had persistent collapse of the epi-glottis. Twenty-three (65.7%) patients were offered targeted surgery based on DISE findings to augment OAT effectiveness. Twenty (57.1%) patients underwent additional medical therapy such as OAT adjustment or cervical positional therapy. Mean AHI was reduced from 37.4 at baseline, to 16.4 with OAT (P\.01), and to 10.7 after post-DISE intervention (P\.78). Conclusion. In patients with incomplete response to OAT, DISE with and without the appliance can identify residual anatomical locations of collapse, which may direct additional medical and surgical treatment options to augment OAT effectiveness. Further work is needed to determine if DISE affects outcomes. Keywords obstructive sleep apnea, continuous positive airway pres