An oral appliance comprises interacting upper and lower dental encasing components formable to overlay portion of the patient's upper and lower teeth. The dental encasings each comprise sheets of a heat softenable and moldable over-mold material on substrates comprising one or more pieces of a stiffening material. The over-mold material is heated to a moderately elevated temperature which will not cause tissue damage when placed in the patient's mouth. While still warm the material can be shaped to conform the patient's teeth but is non-deformable once cooled to body temperature. The stiffening material is non-deformable when heated to the same elevated temperature.
