Boil pasta in salted water until al dente according to package instructions.
Preheat oven to 400 F.
Add vegetable oil to a large skillet and place over medium heat.
Add the onion and cook until translucent and starting to brown, about 5-6 minutes.
Season with salt and black pepper, and cook for 20 more seconds.
Add oregano and garlic, cooking for 1 more minute.
Stir in the mushrooms, reserving a few slices for later to be used as a topping. Cook for 5 minutes, until soft.
Add the cooked pasta and mushroom mixture to a large bowl or pot along with the pasta sauce, 1 cup mozzarella, and all but a few slices of pepperoni (the rest will be used for garnish).
Stir well to combine.
Transfer the pasta mixture to a baking dish and sprinkle over the remaining 2 cups mozzarella.
Garnish with the reserved pepperoni and mushrooms.
Transfer the dish to the oven and cook on the middle rack for 20-25 minutes, or until the cheese is melted and golden brown.