Preheat oven to 160ºC . Cook pasta on stove and drain. Meanwhile, heat oil in saucepan. Add onion and garlic, saute till soft (about 5 minutes). Add tomatoes, oregano, salt and pepper, cook till heated through, about 5 minutes. Combine cooked pasta, sauce and 1.5 cups grated cheese in ovenproof dish. Sprinkle remaining cheese on top.
Bake 15-20 minutes at 180ºC .