Preheat oven to 350 degrees. Spray a 9x13" pan with non-stick baking spray. Set aside. In a mixing bowl, add the cake mix, eggs, water and oil. Mix with a hand or stand mixer, or by hand using a whisk. Mix until the batter is lump-free. Pour the batter into the prepared pan and bake for 30-32 minutes. Remove from oven and let cake cool.

While cake cools, make the filling. In a food processor or blender, crush about 10 OREO's into crumbs. Set aside. With a hand or stand mixer, beat the cream cheese and powdered sugar until creamy. Add the milk and mix. Add the OREO cookie crumbs, and fold together. Lastly, fold in the whipped topping, and mix until just combined.

With the end of a wooden spoon, poke holes into the cooled cake, spacing the holes about 1" apart. Next, add the filling to the top of the cake, pressing the filling gently into the cake to help it go into the holes. Spread evenly.

Cut the remainder of the OREO's into quarters, and sprinkle over the top of the cake. Refrigerate for at least one hour or overnight.