Bake ONLY ¾ of cake mix in a 8" or 9" round cake pan according to package directions. Let cool completely before trimming the top until it's completely level with the rest of the cake. Wrap in plastic wrap and stick in the freezer.
Line the same cake pan with plastic wrap. Spread slightly melted ice cream into the cake pan until it's smooth and flat. Stick in the freezer for at least 30 minutes, ideally at least an hour.
Meanwhile, put about ⅔ cup of fudge topping in a plastic bag (NOT hot...just room temperature) and cut one of the tips off one of the corners. Have that ready to go.
Assemble the cake by quickly, putting the chocolate layer on the bottom of a cake stand then the hardened ice cream layer on top. Cover with whipped topping. Decorate with fudge icing in whatever pattern you'd like. Then break apart Oreos and place them at the base of the cake all the way around.