Make brownie mix according to package directions (see note below)
Pour ½ batter into greased 9x13 pan
Add Oreos, placing them over brownie batter evenly
Spoon marshmallow cream over batter, spreading as evenly as possible (it will not spread super easily, don't worry about it. It doesn't have to be spread over all the brownie batter)
Spread remaining batter over top
Bake at 350* for 20 minutes or until brownies tests done
Note
The ingredients for the brownie mix is based on a Betty Crocker boxed mix. If you use a different brownie mix, these ingredients may vary slightly. Use the instructions on the back of the box you use.