Crush 10 oreos in a bowl with a tart shaper or place in a ziplock bag and crush with a rolling pin. Set aside. Crush remaining 5 cookies and set aside.
Prepare brownies as directed on box and stir in peppermint oil and 10 crushed oreos.
Pour into prepared pan and bake for 18-20 minutes or until center is done. Sprinkle with marshmallows and bake for an additional 1-2 minutes or until marshmallows double in size.
Remove from oven and sprinkle with crushed oreos and m&ms. Let cool completely before cutting.