Boil Pasta: Cook the rigatoni according to package instructions in a large pot of salted water. Drain and set aside.
Preheat Oven: Meanwhile, preheat your oven to 200 degrees Celsius.
Saute: In a large pot, heat the olive oil over medium heat. Add the onion and garlic, and cook until fragrant and soft. Stir in the dried oregano and the chopped vegetables, and cook until the vegetables are tender.
Mix: Add the cooked rigatoni to the vegetable mixture, and then stir in the canned tomatoes. Cook the mixture, stirring frequently, until heated through.
Bake: Transfer the pasta and vegetable mixture into a baking dish, then top with the mozzarella cheese. Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and golden brown.
Serve: Remove the dish from the oven and let cool for a few minutes before serving.