Lightly grease a 9x13 casserole dish with butter or oil spray and set aside. Place marshmallows and peanut butter in a large microwave safe bowl and microwave on high in 30 second increments until the marshmallows are soft and puffy. Add in the cheerios and mix until all the cereal is coated. Press the mixture into the prepared dish and refridgerate for about 30 minutes to set. Cut into 12 bars and store in an airtight container for up to one week.