Line a large sheet pan with parchment paper and set aside.
In a very large bowl combine the Chex, pretzels, Cap’n Crunch, Honey Nut Cheerios, and M&M’s.
In a medium bowl, add the white chocolate chips and vegetable oil. Microwave for 30 seconds, stir, and microwave for another 25 seconds. Stir until melted and smooth.
Pour the melted white chocolate over the snack mix, using a spatula to scrape all the melted chocolate into the mix. Working quickly, gently stir until cereal mix is well coated in the white chocolate, then transfer to the prepared sheet pan and spread into an even layer.
Optional Toppings: Immediately, press a few additional M&M’s on top of the mix for more color and add sprinkles. Add these optional toppings quickly before the white chocolate hardens, or they won’t adhere.
Allow to harden at room temperature or place in the fridge until firm, about 10–20 minutes. Break into pieces and serve.