Bring a large pot of lightly salted water to a boil. Add spaghetti, and cook for 8-10 minutes or until al dente. Drain, and set aside. In a large skillet over medium-high heat, sauté the onions, green peppers and ground beef. Once beef is brown and onions and peppers are softened, add the tomatoes, mushrooms and oregano. Simmer uncovered for 10 minutes. Preheat oven to 350 degrees. Grease a 9x13-inch baking dish. Place 1/3 of the cooked spaghetti into the prepared dish. Top with 1/3 of the meat mixture. Sprinkle with 1/3 cheddar cheese. Repeat twice. Pour soup over casserole. Sprinkle with Parmesan cheese. Bake in preheated oven for 30-35 minutes. Let stand 10 minutes before serving.