In a large bowl, combine cake mix, oil and egg. Add the milk slowly.
Fold in the chopped cookie pieces and sprinkles, reserving some for a topping.
Pour into prepared baking pan, sprinkle a few more sprinkles and crushed Oreos on top. Bake for 25-30 minutes until edges are just turning brown.
Cool for at least 30 minutes so the center sets, before cutting into squares.