In a large skillet, sauté onion and green pepper in butter until tender. Add tomatoes, mushrooms, olives and oregano. Add hamburger if desired. Simmer, uncovered, for 10 minutes.
Place half of the spaghetti in a greased 13x9x2-inch baking dish. Top with half the vegetable mixture. Sprinkle with 1 cup of Cheddar cheese. Repeat layers.
Mix the soup and water until smooth; pour over casserole. Sprinkle with parmesan cheese. Bake, uncovered, at 350° F for 30-35 minutes or until heated through.
Servings: