Spray a 9″ x 13″ casserole dish or cake pan with non-stick spray, set aside.

Measure rice krispies cereal into a large bowl and set aside.

In a large plastic (ziplock type) bag, crush oreos (I used a large, flat bottom mug). You want to have some crushed into small pieces, and some bite size chunks. As an alternative, simply chop the oreo cookies to desired size. Add oreos to rice krispies and stir to combine.

In a large microwave safe bowl, heat butter and marshmallows together (about 3 minutes). They will expand a LOT, so make sure your bowl is big enough. When the marshmallows are puffy and mostly melted, carefully remove from microwave and stir to fully combine the butter, and make sure the marshmallows are fully melted. *Note you are welcome to do this step in a large pot on the stove if you prefer.

Pour melted marshmallows over cereal and oreos and use a spatula sprayed with non-stick cooking spray to quickly combine all the ingredients. As soon as all the cereal is combined, spread mixture into the prepared 9×13″ dish. Use the spatula to press the mixture around the dish, and flatten as needed.

Allow to cool (about 20 minutes) before cutting into squares. Eat and enjoy!