Mix brownie batter according to directions on back of box. 

Pour half of batter into a lined 13x9 pan. Place Oreos flat in batter (don't press down) - 5 across X 4 down.

Place a half marshmallow in between each cookie (so each row should have 5 Oreos and 4 marshmallow halves).

Spoon/drizzle/spread remaining brownie batter over top of Oreos and marshmallows. Make sure top is mostly covered.

Bake according to the directions on your brownie box. Remove from oven. Cool completely before cutting.