Finely crush the Oreos in a food processor. Mix with the melted butter. Line a cookie sheet with parchment paper and press the Oreo mixture into a thin layer. Bake at 350° for 4 minutes. Cool and chill before breaking into pieces.
Melt white chocolate chips in the microwave on 50% power in 30 second increments, stirring between.
Place Chex in a large bowl and pour melted chocolate over the top. Stir and spread out in a single layer on a wax paper lined cookie sheet. Let harden.
Assemble your mix: carefully break crust into bite-sized pieces. Break apart white chocolate Chex. Mix all in a large bowl and stir in M&Ms. Serve in a large bowl or portion into baggies for single servings.