Line an 8x8 baking dish with foil or parchment paper.
Place Oreos in a food processor or blender and pulse until evenly ground.
Add marshmallows and butter to a large, microwave safe bowl, and microwave until butter is melted and marshmallows are soft and puffy.
Fold in crushed Oreos until well combined. Scoop mixture into baking dish and let set, about 10 minutes.
Remove from dish with edges of foil or paper. Cut into squares, serve and enjoy!