Prepare an 8x8-inch pan by covering it with parchment paper with the ends coming up over the sides of the pan; set the pan aside.

Place the Oreo cookies into a food processor and pulse until the cookies are ground into fine crumbs (it may be best to do this step in batches).

Pour the cookies crumbs into a large mixing bowl; set aside.

Heat a large pot over medium heat. Melt the butter in the pot, then add the marshmallows and stir constantly until melted.

Pour the melted marshmallow mixture into the bowl with the cookie crumbs. Stir well to combine.

Press the Oreo and marshmallow mixture into the prepared pan. Set aside to cool for 10 minutes.

Lift the whole sheet of Oreo bars out of the pan and place on a cutting board. Use a sharp knife to cut 9 square bars.

Melt the white melting chocolate according to package directions. Use a spoon, fork, or piping bag to drizzle the melted white chocolate over the Oreo bars in a decorative pattern.

Set the Oreo bars aside for the white chocolate to harden, about 20-30 minutes.