HEAT oven to 325 degree F. Lightly coat a 13x9x2-inch pan with nonstick spray. Stir together the brownie mix, oats, egg, and water in a medium bowl. With floured hands, spread evenly in bottom of prepared pan. Bake for 5 minutes. Layer banana chips, chocolate pieces, marshmallows, nuts over partially baked crust; press down gently. Bake 15 minutes more. Cool in pan on a wire rack; cut into bars. Makes 24 bars.