Line a very large (15x21-inch) sheet pan (or 2 smaller sheet pans) with parchment paper or foil. Set aside. Measure the Rice Chex cereal and add to a very large bowl.
Add Oreos to a high-powered blender or food processor (whole cookies—don’t remove the cream filling). Blend or pulse until they are all crumbs. Measure 1 level cup of crumbs (135g) and set aside the rest of the crumbs for later.
Add milk chocolate chips and 2 teaspoons coconut oil to a large microwave-safe bowl. Microwave in bursts of 20 seconds, stirring between each burst for 20 seconds, until chocolate is melted and smooth. Add the 1 cup crushed Oreos to the completely melted chocolate and stir until combined. Pour chocolate and Oreo mix in the bowl with the cereal. Use a spatula to scrape every bit of chocolate into the cereal. Gently stir cereal until well coated in the chocolate and Oreo mix. Then, transfer the cereal to the prepared pan(s) in an even layer.
Add white chocolate chips and remaining 1 teaspoon coconut oil to a large microwave-safe bowl. Microwave in bursts of 20 seconds, stirring between each burst for 20 seconds, until white chocolate is melted and smooth. Line a cup with a resealable plastic bag and pour the melted chocolate into the bag. Seal the bag with no air and cut off the tip of the bag. Pipe white chocolate generously over the chocolate-coated cereal until you’ve used all the white chocolate.
Working quickly (white chocolate hardens fast!) sprinkle on the optional chocolate chips and mini Oreo cookies. Sprinkle reserved remaining cookie crumbs over the mix. Let the mix harden at room temperature until set, about 1 hour (or in the fridge for 20 minutes). Break the snack mix apart and enjoy! This mix is best enjoyed within 2 days.
Notes
Nutrition Note: Nutritional information includes the optional mini Oreos and chocolate chips.Storage: Store leftovers in an airtight container or gallon-size bag in a cool, dry place; best enjoyed within 2–3 days, but it can last up to a week. To freeze, use a freezer-safe container or bag, remove extra air, and freeze for up to 2 months.