The invention is an oral appliance for prevention of sleeping problems including snoring, sleep apnea and bruxism. Specifically, the appliance alters the position of the user's mandible, which is known as a method for reducing the restriction of the flow of air through the pharyngeal passageway. The appliance is a one-piece device molded from a flexible polymer such as Kraton®. It includes an upper maxillary tray and a lower mandibulary tray. Both upper and lower trays include inner and outer walls which increase contact area with the teeth. The hinge mechanism of the device includes a positive positioning system comprised of opposed interlocking ridges. The ridges serve to create offset between the position of the upper tray and lower tray relative to each other, therefore advancing the user's mandible. Additional features include airflow posts to keep the mouthpiece from sealing completely, a cleft contour, and a tooth retention tab.