In a large mixing bowl, whip the heavy cream until soft peaks form.
Gradually add the powdered sugar and vanilla extract, continuing to whip until stiff peaks form.
Gently fold in the dry funfetti cake mix until well combined.
In a 9x13 inch baking dish, dip each Oreo cookie in milk for about 2 seconds, allowing excess milk to drip off.
Lay the dipped cookies in a single layer on the bottom of the dish.
Spread a layer of the whipped cream mixture over the cookies, then sprinkle with rainbow sprinkles.
Repeat the process, layering dipped Oreos and whipped cream mixture, until all ingredients are used, finishing with a layer of whipped cream on top.
Sprinkle additional rainbow sprinkles over the final layer.
Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight for best results.
Slice and serve chilled.