In a large mixing bowl beat peanut butter and oil with an electric mixer on low to medium speed for 30 seconds. Add brown sugar, eggs, baking powder, and baking soda. Beat until combined, scraping sides of bowl occasionally. Beat in flour; then beat in the milk. Using a wooden spoon, stir in rolled oats, chocolate pieces, and peanuts. Spread batter in prepared pan.
Bake in a 350 degree F oven for 25 to 30 minutes or until a wooden toothpick inserted near center comes out clean. Cool completely in pan on a wire rack.
Cut into triangles, circles, and squares that are about 2-1/2 inches across (reserve end pieces for nibbling). Decorate shapes with purchased frosting and candies so they look like drums, presents, and trees. Makes about 16 bars.