Pour pudding mix into a bowl. Add 2 cups of your milk and whisk together for 2 minutes, breaking up all lumps. Once pudding has thickened, place in refrigerator for 10 minutes.
While pudding is resting, open your package of Oreos. Dunk each Oreo in your remaining one cup of milk, and then place the Oreos side-by-side in the bottom of your pan. Repeat until the entire bottom of the pan is covered in a layer of Oreos.
Remove pudding from refrigerator and spread half to 2/3 of it in a thin layer overtop your layer of Oreos.
Repeat your Oreo-dunking process, this time placing the dunked Oreos atop the layer of pudding.
Spread the rest of your pudding atop your Oreos. Cover this layer of pudding with a thick layer of Cool Whip.
Drizzle the pan with chocolate syrup and refrigerate for at least 2 hours, or overnight.
Slice and serve!