Line an 8×8-inch pan with parchment paper and lightly grease it. This fudge is sticky so be sure to do both.
In a microwave-safe bowl, combine the sweetened condensed milk, white chocolate, and butter. Microwave in 30-second intervals, stirring until fully melted and smooth.
Stir in half of the crushed Oreos, half of the M&Ms, and sprinkles if desired, mixing them evenly into the fudge base.
Pour the mixture into the prepared pan and smooth it out. Sprinkle the remaining Oreos, M&Ms, and Christmas sprinkles on top. Use as many sprinkles as you’d like!
Chill in the fridge overnight. Cut into squares and enjoy!