Preheat the oven to 350°F/180°C. Spray a 9x13 inch casserole dish with non stick spray/ cooking oil and set aside.
Cook pasta in a large pot of salted water according to the package directions.
While the pasta cooks, heat olive oil over medium high heat in a skillet. Add the ground beef and cook, breaking it apart. Season with oregano, salt, and pepper.
When the meat is almost browned and fully cooked, add the onion and garlic and sauté for one minute. Then add the mushrooms and green peppers, and cook for a few minutes until the vegetables are tender.
Stir in the pizza sauce and cook for 1 minute to warm through.
Remove the sauce from the heat and stir in the olives and pepperoni slices, saving 12-15 slices of the pepperoni for the topping. Add the cooked and drained pasta and stir well.
Transfer the mixture to the prepared casserole dish, top with shredded mozzarella and reserved pepperoni. Bake in the preheated oven for 15 minutes or until the cheese has melted and the casserole is bubbly.
Serve, garnished with fresh parsley if desired.