A customizable mandibular advancement device for use in treating snoring and obstructive sleep apnea is provided. The device comprises upper and lower trays adapted to be fitted to a person's maxillary and mandibular arches. The upper tray comprises a stop member, which when engaged with a fin located on the lower tray, inhibits the posterior movement of the user's mandible thereby maintaining the mandible in an advanced position. Both the upper and lower trays comprise a core that is overmolded by a thermoplastic material that has a much lower softening temperature than the core.