An oral treatment device that emits light onto surfaces of a user's teeth. In one aspect, the oral treatment device includes an intraoral mouthpiece. The mouthpiece comprises a lamp support structure comprising a lamp support surface and is formed of a first material. A flexible lamp is mounted to the lamp support surface so that a light emitting surface of the flexible lamp assumes a concave curvature, thereby forming a lamp assembly. A guard component is overmolded to the lamp assembly. The guard component is formed of an elastomeric material having a hardness that is less than a hardness of the first material. In one embodiment, the oral treatment device may be a teeth whitening device.