In a large saucepan, melt butter on medium heat. Add in marshmallows and stir constantly until melted. As soon as melted, remove from heat. Pour cereal into marshmallow and butter mixture. Mix until all cereal is well combined with marshmallow mixture. Spread mixture into a baking sheet or pan lined with wax or parchment paper. If adding crushed Oreos, spread over crispy treats and lightly press into crispy treats with buttered spatula. Allow to set for at least an hour, then cut, serve, share and enjoy.