Prep an 8x8 inch pan by spraying with nonstick cooking spray or lining with parchment paper.
Use a food processor to crush the Oreo cookies. Don't crush all of them! Set 4 to 6 cookies aside to use as a topping to your Oreo bars. You'll probably have to crush the Oreos in batches, rather than the whole package in one go. Pour the crushed Oreo cookies into a large bowl.
In a large pot over low heat, melt butter and marshmallows. As they melt, stir them regularly so that the marshmallow mixture is smooth and the butter is evenly incorporated.
While the melted marshmallows are still hot, quickly pour them into the Oreo crumbs and stir. The marshmallow goodness is going to harden fast, so work fast!
Press the Oreo marshmallow mixture into the prepared pan, making sure to get it into the corners and packed down well. Press the mixture using either your hand or the bottom of a measuring cup. Allow the bars to set up for 10-15 minutes before going on to the next step.
While the bars are setting, melt the white chocolate in a microwave-safe bowl in 30-second increments, stirring frequently. Then, drizzle the chocolate on top of the bars and top with the Oreos you set aside earlier, cut or chopped for the look you prefer.
Use a sharp knife to cut the pan into bars, and enjoy!