Heat a large and deep skillet or medium to large pot over low heat.
Add the butter and heat until melted.
Add the marshmallows and stir occasionally until the marshmallows are melted, approximately 5 minutes.
Meanwhile, chop the cookies into small chunks.
Remove the pot from the heat. Add the Oreos and rice cereal and stir to incorporate.
Pour the rice mixture into a 9x13 inch baking dish that has been sprayed with nonstick spray.
Use your hands to press the mixture across the baking sheet in an even layer, all the way to the edges and corners.
Let sit until the marshmallow stiffens.
Cut the treats either into rectangles or squares as desired. Store in an airtight container for up to three days.
Notes
I like the treats to be softer, which means more marshmallow. If you want the treats firmer with little marshmallow, reduce butter to 4 Tablespoons and marshmallow to 6 cups.