In a small skillet, heat the oil over low heat. Drizzle it over the popcorn in a large baking pan, tossing it well, and bake the popcorn in a preheated 300 degree oven, stirring once or twice, for 10 to 15 minutes, or until it is crisp and very hot. Sprinkle the Parmesan over the popcorn, tossing it, and season the popcorn with salt if desired.