Place Oreos in food processor and pulse until ground.
Melt marshmallows and butter in microwave until puffed, about 1 1/2- 2 minutes.
Remove marshmallow mixture and mix with ground Oreo Cookies.
Stir to combine then transfer to a foil lined 8×8 inch baking pan.
Let stand for 10 minutes.
Remove bars out of pan with edges of foil and cut into squares.