Line a 9x9 (or 8x8 or something around there) pan with parchment paper and set aside. 

Place cereal in extra large mixing bowl.  Add 1 cup mini marshmallows.  Coarsely chop oreos and add to bowl with cereal.  Melt butter in a large pan on stove top.  Add marshmallows and stir on low heat until melted and smooth.  Remove from heat and stir in extract.  Immediately pour over cereal mixture and stir well to combine.  Transfer cereal mixture over to prepared pan and gently press down with spoon.  Let cool completely and then cut into squares.