Bring a large pot of water to boil for the spaghetti. Cook the noodles 1 minute shy of minimum package directions.
Meanwhile, heat olive oil in a wide, deep skillet over high heat. Add onions and peppers. Cook, stirring occasionally, until the vegetables are limp and reduced in size, about 10 minutes. Turn the heat down as needed to prevent burning.
Add ground beef. Cook it, breaking it up with a fork, until all traces of red disappear. Drainany fat from the pan.
Add tomatoes, tomato sauce, oregano, salt and pepper. Bring to a boil. Reduce heat and simmer 20 minutes if you have the time.
Heat overn to 350 degrees.
Combine cooked noodles, hamburger mixture and about half the cheese in a 9-by-13 inch casserole pan. Spread it out evenly, then top with remaining cheese. Bake 20 minutes. Let stand 5 minutes before serving. Leftovers reheat beautifully in the microwave.