Preheat oven to 180 degrees C. Grease a 20x30cm pan.
In a medium bowl, mix together the oats, brown sugar and white sugar. Stir in the melted butter until well blended. Pat the mixture evenly into the bottom of the prepared pan.
Bake for 15 minutes in the preheated oven until toasty. In the microwave or in a heat-proof bowl over simmering water, melt chocolate chips and peanut butter together, stirring occasionally until smooth. Spread over the baked crust. Let cool before cutting into squares.