Preheat oven to 350°F and line a 9x13-inch pan with parchment paper or spray with non-stick cooking spray.
In the bowl of a stand mixer, fitted with the paddle attachment, or with a hand mixer in a large bowl, beat together the cake mix, 1 box of chocolate pudding mix, sour cream, oil, eggs, vanilla and water on medium speed until well combined, about 2 minutes. Be sure to scrape the bottom of your bowl as needed.
Coarsely chop 10 Oreos and fold the chopped Oreos in the cake batter.
Spoon batter into the prepared pan and use an offset spatula or spoon to spread the batter into an even layer.
Bake in preheated oven for 30-35 minutes, or until the top of the cake springs back when lightly touched.
Allow cake to cool for 10 minutes. Using a smoothie straw or the handle of a wooden spoon, poke holes across the cake, spaced every inch or so, halfway into the cake.
In a medium bowl, whisk together the remaining box of chocolate pudding mix and milk for 2 minutes. Immediately pour the pudding evenly over the cake, making sure the mixture gets into the holes. Allow cake to cool completely.
Finely chop 8 Oreos and gently fold them into the whipped topping. Evenly spread the whipped topping onto the cake. Cover and chill for at least 4 hours.
Just before serving, coarsely chop the remaining 8 Oreos and sprinkle them onto the cake and if desired, drizzle with chocolate fudge sauce.
Notes
If you can locate the Oreo Jell-o instant pudding mix in your area, please feel free to use that instead of the chocolate pudding.