In a medium saucepan, melt the 1/4 cup of butter over low heat. Be careful not to let it brown; you just want it melted.
Once the butter is melted, add the entire 16oz bag of miniature marshmallows to the saucepan. Stir continuously until the marshmallows are completely melted and smooth.
Remove the saucepan from heat. In a large mixing bowl, add the Hershey's Cookies & Cream Cereal. Pour the melted marshmallow mixture over the cereal. If you’re using them, add the 1 cup of crushed Oreo cookies at this stage. Stir until the cereal is evenly coated with the marshmallow mixture.
Transfer the mixture into a greased or parchment-lined 9x13 inch baking dish. Use a spatula to press the mixture evenly into the dish. Make sure to pack it down firmly but gently to avoid crushing the cereal too much.
Allow the treats to cool at room temperature for at least 30 minutes. Once they are set, cut them into squares or rectangles.
