Spray a 9 by 13 inch baking dish with cooking spray or line with parchment paper. Set aside.
In a large saucepan, melt the butter over low heat. Stir in the marshmallows and allow them to melt with the butter until smooth. Turn off the heat.
3 tablespoons butter, 6 cups mini marshmallows
Remove from the heat. Pour in the cereal and stir to coat completely. Stir in the Oreos and stir so they are evenly distributed throughout the mixture.
5 1/2 cups puffed rice cereal, 20 Oreo cookies
Pour the mixture into the prepared dish. Press down with more parchment paper, or grease your washed hands and use them to push the mixture into the baking dish.
Allow to cool. Remove from the pan and cut into squares. Enjoy or store in an airtight container for up to 4 days.