An oral appliance (1) is provided for the treatment of snoring and obstructive sleep apnea. The oral appliance (1) includes an upper tray (10) adaptable to conform to a user's maxillary dentition and a plurality of lower trays (40), each of the lower trays (40) adaptable to conform to the user's mandibular dentition. At least some of the lower trays (40) are structured to engage the upper tray (10), each of the lower trays (40) are structured to impart a different fixed amount of mandibular advancement.