Preheat the oven to 350.
Chop up the Oreos into large pieces.
Prepare the chocolate cake by following the box directions.
Coat sides and bottom of a 9x13 dish with shortening.
Pour batter into the prepared dish.
Bake this for 30 minutes or until a toothpick can be inserted in the middle and can come out clean.
Whisk together milk and pudding mix until it is smooth.
Poke holes all around the top of the cake.
Pour the pudding evenly over the cake to allow it to soak into all of the holes.
Sprinkle the top with about half of the chopped Oreos.
Allow this to cool completely.
Top with the whipped topping and sprinkle with the remaining chopped cookies.
Refrigerate for at least 1 hour.