Place Oreos in food processor and pulse until ground.
Melt marshmallows and butter in microwave until puffed, about 1 1/2- 2 minutes. Remove and pour in ground Oreo Cookies. Stir to combine.
Transfer to a foil or wax paper lined 8×8 inch baking pan. Let set up for 10 minutes.
Remove bars from the pan and cut into squares.