The oral appliance, either a mouthguard or an orthodontic device, includes a base member having a U-shaped form corresponding to a row of teeth of a user. The base member has inner and outer flanges interconnected by a web which collectively define upper and lower channels within which respectively the upper and lower rows of teeth of the user are received. Upper and lower teeth engaging elements are positioned within the upper and lower channels. The teeth engaging elements are made of EVA which softens at a temperature of 90–95° C. The appliance may also include a tongue tag (25) and breathing apertures (30).