Preheat oven to 350 degrees. Spray an 11x7 inch baking pan with cooking spray and set aside.
Crush Oreo cookies in a large, resealable zip top bag with a meat mallet or large soup can. (You could also crush them using your blender or food processor.)
Combine the crumbs and melted butter in a bowl, mixing until combined. Press mixture into the bottom of prepared baking pan.