Preheat oven to 325 F. 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 1/2 cups grated cheese in ovenproof dish. Sprinkle remaining cheese on top.
Bake 15-20 minutes at 350 F.