In a 9x13 baking dish, bake the chocolate cake according to package directions. Once it is cooked through, remove it from the oven and allow it to cool for 10-20 minutes.
While the cake is baking, mix the pudding mixture with the milk according to the package directions. Pudding may be a bit lumpy because of the cookie pieces (see notes below). Refrigerate it until the cake cools a bit.
Once the cake cools, use the rounded end of a wooden spoon to poke holes through the entire cake by pressing straight down through the cake repeatedly.
Pour the pudding mixture all over the cake, and spread it out to ensure it seeps into all of the holes.
Next, spread the whipped topping all over the top of the pudding, into one even layer.
Sprinkle the crumbled Oreos over the top of the whipped cream layer. Cut and serve immediately, or cover and refrigerate for 3-4 days.