An oral appliance is a small plastic device that fits over the upper and lower teeth, similar to an orthodontic retainer or mouth guard. The appliance is worn in the mouth during sleep to prevent the soft tissues of the airway from collapsing and obstructing the airway. Oral appliances work by slightly advancing the lower jaw which in turn moves the base of the tongue forward and opens the airway to allow improved breathing and reduced snoring and apnea during sleep.