Melt marshmallows and butter in a large pot over low heat, stirring constantly till smooth.
Add Rice Krispies, Oreos, and sprinkles. Stir until everything is well coated in the marshmallow mixture.
Transfer to a buttered 13 x 9 inch baking dish. Using a buttered spatula or hands, press evenly into the dish. Cool for about 20 minutes. Cut into squares.