Preheat oven to 350 degrees. Line 9x13 pan with parchment paper and spray with nonstick cooking spray. Set aside.
Stir graham cracker crumbs together with melted butter until moist. Press this into the bottom on the prepared pan creating an even thin layer. Pour sweetened condensed milk overtop, spreading to cover the bottom crust evenly. Sprinkle with mini chocolate chips, marshmallow bits and oreos.
Bake 15-20 minutes or until edges are golden and bars are set. Cool to room temperature, cut into bars and serve.