A oral appliance (i.e., a dental mouth guard) configured so as to fit in the mouth and over at least one (e.g., the upper) set of teeth of a user during sleep to prevent bruxism and the grinding together of the user's upper and lower sets of teeth. The mouth guard has a relatively soft heat responsive and arcuate shaped upper impression tray that is molded over a relatively hard and arcuate shaped lower chassis. The mouth guard is removably received within and transported by a fitting tray to be submerged in boiling water. After it is heated, the fitting tray relocates the mouth guard to the user's mouth where a biting force is applied by the user to the upper impression tray so that an accurate and custom impression of the user's teeth is made therewithin. The mouth guard is now removed from the fitting tray which is then discarded.
