Preheat oven to 350 degrees F. Spray a 9x13-inch baking dish with non-stick cooking spray. Cook spaghetti according to package directions.
Heat butter in a large non-stick skillet over medium-high heat until hot. Sauté onion and pepper until tender. Add ground beef and cook until beef has browned, drain.
Stir in tomatoes, mushrooms, olives and oregano. Bring to a boil and reduce heat to medium. Simmer for 10 minutes.
Place half of spaghetti in the prepared baking dish. Top with half of vegetable-meat mixture. Sprinkle with half of Cheddar cheese. Repeat layers. Combine soup with 1/2 cup water; pour over casserole. Sprinkle with Parmesan cheese. Bake uncovered for 30 to 35 minutes or until thoroughly heated.