Combine ground beef, onion, garlic, and green pepper in a large saucepan over medium-high heat. Cook and stir until meat is browned and crumbly and vegetables are tender, 5 to 7 minutes. Drain grease.
Stir diced tomatoes, tomato sauce, and tomato paste into the pan. Season with oregano, basil, salt, and pepper. Simmer spaghetti sauce for 1 hour, stirring occasionally.