In a large microwave safe bowl, combine the oil and ranch mix. Stir. Add the popcorn, pretzels and cashews and toss to evenly coat the mixture. Heat on high for 1 minute, then stir, mixing from the bottom (the oil has a tendency to pool at the bottom). Heat for an additional 1 minute, then add the Parmesan. Toss to coat evenly. Serve immediately or store in an airtight container in the fridge for up to 3 days.