To make Base: Combine oats and sugars in large bowl. Add melted butter and mix well. Press into greased 9X13" pan and bake in preheated 350F oven for 15 to 20 minutes. Cool for 20 minutes. Meanwhile, prepare topping. Melt together chocolate chips and peanut butter, stirring to blend. Spread on top of cooked cookie base and refrigerate for 1 hour. Cut into squares before serving. Yield: 3 dozen