PLACE 2 flat pie crusts on parchment-covered full sheet pan. Bake in 425°F-conventional oven for 10 to 12 minutes or until golden brown. Set aside to cool.
SPREAD 1/4 cup of the melted chocolate onto each pie crust; top with 1-1/2 cups of the OREO Pieces and 1/2 cup each of the marshmallows and walnuts. Drizzle each pizza with additional 1/4 cup chocolate. Cool 30 minutes or until chocolate is set.
CUT each pizza into 8 wedges to serve.