Lightly coat a 9x13-inch baking pan with nonstick spray and set aside. Measure out the cereal into a bowl, and open the bag of marshmallows. Set both aside.

In a large pot set over low heat, add the butter and melt. Once melted, add the mini marshmallows and stir to coat with the butter. Stir frequently to melt the marshmallow.

Once completely melted and smooth, remove from heat and stir in the cereal all at once. Work quickly to stir and coat the cereal, leaving no loose bits of cereal or pockets of marshmallow. Stir in the Oreo pieces until evenly mixed.

Scoop the marshmallow mixture into the prepared pan. Lightly press down with a greased hand or spatula to reach to the edges of the pan; it's OK if it doesn't fully reach.

Let cool for 1 hour, then cut and serve. Store covered with plastic wrap, or in an airtight container, at room temperature.