Line 2 rimmed baking sheets (or 1 very large baking sheet) with wax paper. Set aside.
Add rice chex, cheerios, pretzels, peanuts and m&m candies in a very large bowl and stir to combine.
In a large microwave-safe bowl, add white chocolate, vegetable oil (if using), and microwave on MED power (50%) for 1 minute 30 seconds, stirring every 30 seconds.
If needed, microwave again, in 15 second intervals, stirring after each cooking interval, until chocolate is JUST melted.
Pour melted chocolate over ingredients in large mixing bowl and use a rubber spatula to gently stir, until all ingredients are coated.
Spread out onto prepared baking sheet in an even layer. Let sit for 1 hour, or until set and hard. You can refrigerate the baking sheets to speed up this process if you'd like. Break apart into pieces and enjoy!