Grease an 8 or 9-inch baking dish with non-stick cooking spray and set aside.
In a medium pot, melt marshmallows and butter over medium heat until melted. Stir in baking pieces until melted into marshmallow mixture and remove from heat.
Stir in Oreo pieces until cookies are completely coated. Press mixture into prepared dish and allow to cool before cutting into bars and serving.
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