Crush the 6 Oreo cookies into a mix of big and small pieces (you can do this either in a food processor or with a resealable bag and a rolling pin or mallet) then set them aside
In a large pan over low heat, melt the butter
Add the mini marshmallows and leave them alone for 1-2 minutes until they melt - no need to stir, just make sure the heat is low
Add in the crispy rice cereal and mix; mix slowly and gently so the cereal does not spill over the sides of the pan
Stir in the crushed Oreo cookies; I like to stir in an extra ¼ cup of mini marshmallows as well so they stay whole, but that's totally optional
Transfer to a parchment paper lined heatproof dish of any kind and press flat using a spoon or spatula (see Note 1)
When it's cool enough, use parchment paper and your hands to press it flat again so it’s an even layer
Note 1: The parchment paper in this recipe is not strictly necessary, but it not only helps when pressing the bars into a flat layer, but also with removing the bars from the baking dish before cutting them. As a bonus, it makes cleaning the baking dish easier because the marshmallows are sticky.Nutrition Facts are calculated automatically and may be inaccurate.