In a large ziploc bag, place the Oreos in, seal the bag, and with a rolling pin, crush the Oreos until they're smaller pieces. Set aside.
In a large pot, melt 3 tbsp. (42g) butter. Once butter has melted, add in the bag of marshmallows and stir until all the marshmallows have melted then remove from heat. Gently fold in the rice cereal then fold in the Oreo's.
Pour all the contents into the prepared pan and spread out evenly. To make this faster/easier, I usually spray my hands with some cooking spray and use my hands to press down on the mixture until it's evenly spread out.