PLACE milk, mousse mix and peanut butter in bowl of electric mixer fitted with wire whip attachment. Beat on medium speed 2 minutes. Scrape side of bowl. Beat an additional 4 to 5 minutes or until mixture forms stiff peaks. Fold in OREO Pieces.
SPREAD about 3-1/2 cups of the mousse mixture into each crust; cover.
REFRIGERATE at least 1 hour.