In a large skillet, saute onion and green pepper in butter until tender. Add tomatoes, mushrooms, olives and oregano. Add ground beef, if desired. Simmer, uncovered, for 10 minutes. Place half of the spaghetti in a greased 13" x 9" x 2" baking dish. Top with half of the vegetable mixture. Sprinkle with 1 cup cheddar cheese. Repeat layers. Mix the soup and water until smooth; pour over casserole. Sprinkle with parmesan cheese. Bake, uncovered, at 350 degrees for 30-35 minutes or until heated through.