Preheat oven to 350°F and spray a 9x13-inch baking pan with nonstick spray. Bake cake per instructions on the box.
15.25 ounces chocolate cake mix, 1¼ cups water, ⅓ cup vegetable oil, 3 large eggs
Transfer cooked cake to a cooling rack and let it cool 10 minutes.
Use the rounded handle (diameter ¾-1 inch) of a wooden spoon or rubber spatula to poke holes in 1-inch intervals in the cake.
In a medium bowl, whisk together dry pudding mix and milk until it just begins to thicken, approximately 1½ minutes. It should be slightly thickened.
8.4 ounces instant Oreo pudding mix, 3 cups milk
Pour the pudding over the warm cake and specifically aim for the holes.
Use an offset spatula to spread the pudding over the entire cake. Gently push down on the pudding as you spread it over the cake.
Cover the cake with plastic wrap and refrigerate until completely cool, about 1 hour.
When the cake is cool, spread the whipped topping over the pudding layer.
8 ounces whipped topping
Recover the cake with plastic wrap and refrigerate until ready to serve.
While the cake is in the refrigerator, place half (about 20) cookies in a Ziplock bag and crush them or use a food processor.
14.3 ounces Oreo cookies
When ready to serve the cake, sprinkle the whipped topping layer with the crushed and whole Oreos.