Prepare the brownie mix as directed on the back of the package, omitting the water or milk called for on the box. Mix in the ice cream and hot fudge sauce.
Spread half of the brownie batter in a parchment lined 9x13 baking dish. The mixture will be very thick. Lay out the Oreos to cover the brownie batter. Pour remaining batter over the top of the Oreo cookies carefully so that you are covering each Oreo cookie.
Bake for 40-45 minutes or until a toothpick comes out mostly clean. These will stay nice and chewy for up to a week if you keep them tightly covered.