Grease a large mixing bowl, a 9-inch square pan, and a large silicone spatula with butter or baking spray.
Pour the OREO O's Cereal and OREO Cookie pieces into the large mixing bowl.
You can reserve some of the cookie pieces to sprinkle over the top of the bars if desired.
Set aside.
Begin melting the butter in a medium-large saucepan set over low heat.
Toss in the marshmallows and stir occasionally until about 75% of the marshmallows have melted.
Remove the pan from the heat and stir vigorously, smashing the marshmallows, allowing them to melt until smooth.
Pour the melted marshmallows over the top of the OREO cereal and cookie pieces.
Toss to coat evenly.
Gently press the cereal mixture into the prepared pan.
Sprinkle any reserved cookie pieces over the top if using.
Let cool for about an hour before cutting into 16 squares.
Notes
Store your OREO O's Cereal treats in an airtight container at room temperature for up to 5 days.
Microwave Directions:
Heat the butter and marshmallows on high power, in a large mixing bowl, for 1 minute.
Remove and stir vigorously until melted, smashing the marshmallows to help them melt. If needed, heat for additional 15-second increments, stirring vigorously after each, until melted and smooth.
Pour over cereal then follow the instructions above.