Heat the oil in a large pot over medium-high heat. Add the popcorn, garlic powder, oregano, and chili powder, and shake it all about. Heat until the popping starts, shake it every once in a while, and take it off the stove right when the popping quiets down to almost nothing.
Drizzle over the butter, if using, replace the lid and give it a few more shakes to evenly distribute the butter. Sprinkle with salt and the Parmesan, if using, cover, and shake it up again. Pour into a bowl and eat warm.
Notes
Leftover popcorn can be stored in an airtight container for up to 5 days. If it's buttered, it should be eaten within a day, and it will lose some of its crunchiness.