Preheat oven to 350 degrees Fahrenheit. Lightly spray an 8x8" or 9x9" baking dish with nonstick cooking spray, then line the bottom of it with Oreos (I used 16) so they are side by side, touching, and fill the entire bottom of the dish.

Make brownie batter by mixing together brownie mix, egg, vegetable oil and milk in an electric mixer. Spread carefully over Oreos. Crush 7 more Oreos in a zip top bag and sprinkle over the top of the batter. Add Halloween sprinkles. Bake for approximately 30-35 minutes. Allow them to cool completely before cutting into bars.