Place the entire pack of Oreos in a food processor and pulse until ground into crumbs.
Put the butter and marshmallows in a large microwave-friendly bowl. Put in the microwave and heat on high for 1-2 minutes, or until puffy.
Marshmallows can set very quickly once microwaved, so promptly add the blended cookie crumbs. Combine thoroughly. If the mix hardens quickly then add a tablespoon of water to help soften it up a bit.
Line a 9x9 inch square pan with parchment paper and a little bit of butter. Transfer the Oreo and marshmallow mixture from the bowl to the pan. Press in thoroughly with a spatula to help spread the mix evenly over the parchment paper, and work it into the corners of the pan.
Leave to set for 10-15 minutes. Remove bars from pan and cut into desired number of pieces.
Either serve up immediately or set in the fridge until needed.