Lightly grease a 9x13-inch baking dish with a thin swipe of butter or nonstick spray, then wipe out any excess with a paper towel so the Oreo crust releases cleanly.
Place 36 Oreo cookies (cream included) in a large zip-top bag and crush with a rolling pin until fine crumbs with a few small chunks remain, or pulse in a food processor without turning them into dust.
36 Oreo cookies
In a medium bowl, combine the Oreo crumbs with the melted butter and stir until evenly moistened and the mixture looks like wet sand. Press firmly into an even layer in the prepared baking dish, using the bottom of a measuring cup to compact it, especially in the corners.
36 Oreo cookies, 6 tablespoons unsalted butter
Refrigerate the crust for at least 15–20 minutes to allow the butter to firm up and create a sturdier base.
In a large mixing bowl, beat the softened cream cheese with a hand mixer for 1–2 minutes until smooth and fluffy. Add the powdered sugar, milk, and vanilla extract and beat again until creamy with no lumps, scraping down the bowl as needed.
8 ounces cream cheese, 1 cup powdered sugar, 2 tablespoons milk, 1 teaspoon vanilla extract
Gently fold 8 ounces of the thawed whipped topping into the cream cheese mixture with a spatula until fully incorporated and light and fluffy, being careful not to deflate the mixture.
16 ounces frozen whipped topping
Remove the chilled Oreo crust from the refrigerator. Spoon the cream cheese mixture over the crust and spread it into an even layer all the way to the edges, taking care not to disturb the crust. Return the pan to the refrigerator while you prepare the pudding.
In a clean bowl, whisk together the 2 boxes of instant chocolate pudding mix and the cold milk for about 2 minutes until slightly thickened and smooth. Let it sit for 4–5 minutes to continue thickening to a soft-custard consistency.
2 boxes instant chocolate pudding mix, 3 1/4 cups cold milk
Gently pour or spoon the thickened chocolate pudding over the chilled cream cheese layer and spread evenly from edge to edge with a spatula, using a light touch so the layers stay distinct.
Let the pudding layer sit for about 5 minutes, then spread the remaining 8 ounces of whipped topping evenly over the top as the final layer, creating swirls if desired.
16 ounces frozen whipped topping
Sprinkle the top with mini chocolate chips and crushed Oreos. Cover the dish with plastic wrap or a lid and refrigerate for at least 4 hours, or overnight for the best flavor and cleanest slices.
1/2 cup mini chocolate chips, 4-6 Oreo cookies
Use a sharp knife to cut the dessert into 15–18 squares, wiping the knife between cuts for neat layers. Serve chilled straight from the refrigerator.