Sauté onion, green pepper, and garlic in a large, heavy, ovenproof skillet in 1/4 cup olive oil until vegetables are tender. Add tomatoes, tomato sauce, basil, oregano, salt, and pepper, stirring well. Cover and bake at 350° for 1 hour and 30 minutes, stirring occasionally.
Sauté chicken livers in remaining olive oil for five minutes. Combine chicken livers and sauce, stirring well. Cover and simmer an additional 10 minutes.
Prepare spaghetti according to package directions. Serve sauce over spaghetti. Sprinkle with Parmesan cheese, if desired.