Prepare cake mix according to package directions for a 9x13 cake. Once cake comes out of the oven, allow it to cool for just a couple of minutes.
Then, with a wooden spoon handle, a spatula handle, or some other similarly-sized object, begin poking holes in the warm cake. You want the holes to be BIG so that the pudding has plenty of room to get down in there. Be sure to poke right down to the bottom of the cake.
In a medium bowl, whisk together pudding mix with 4 cups of milk. Whisk until all the lumps are gone.
Pour pudding over cake. Taking care to pour it right into the holes as much as possible.
Spread it all out and using the back of the spoon, gently push pudding down into the holes.
Put the cake into the fridge to set and cool (about 2 hours).
Once your cake has completely cooled, spread on whipped topping. Spread it out evenly over the pudding layer.
Spread crushed Oreos onto the top of the cake.
Adding the crushed Oreo's immediately before serving will ensure the cookie bits are crunchy when you serve it. Keep refrigerated.