To start, prepare a 9x9-inch or 9x13-inch pan by generously greasing it with softened butter. In a large mixing bowl, combine the crispy rice cereal with roughly chopped Oreo cookies. Set this mixture aside.

Next, in a large pot, melt butter over medium heat. Once melted, stir in the mini marshmallows, cooking and stirring until completely melted. Remove the pot from heat and pour the melted marshmallow mixture over the cereal and Oreo mixture. Stir well until everything is evenly coated.

Scoop the mixture into the prepared pan and press it down evenly. Let the bars sit at room temperature for about an hour. Once set, turn the pan over onto a cutting board and shake gently to release the treats. Use a knife to cut them into squares.