Start by lining a large sheet pan with parchment paper. This keeps the sticky, chocolate-covered mix from fusing to the pan once it hardens. Set the pan aside so it's ready the moment your mix is coated.
In a very large bowl, add the Chex, pretzels, Cap'n Crunch, Cheerios, and M&M's. Gently toss everything together so the textures and colors are evenly distributed. Try not to crush the cereal pieces while mixing.
In a microwave-safe bowl, combine the white chocolate chips with the oil. Microwave in short 25-30 second bursts, stirring well between each one, until it's completely smooth. Going slowly here prevents the chocolate from scorching or turning grainy.
Pour the melted white chocolate over your cereal mixture, scraping the bowl clean with a spatula. Working quickly, gently fold everything together until each piece is lightly coated. Speed matters here since white chocolate firms up fast.
Transfer the coated mix onto your prepared sheet pan and spread it into an even layer. Immediately press a few extra M&M's on top and scatter your sprinkles over everything. Do this fast, before the chocolate sets and stops being sticky.
Let the pan sit at room temperature to set, or pop it in the fridge to speed things up, about 10 to 20 minutes. Once firm, break the slab into bite-sized clusters and it's ready to serve.