(fresh mozzarella is best, sliced)
In a large bowl, combine the beef, egg. Parmesan, ketchup, oregano, Worcestershire, salt, and pepper. Mix together using your hands just until combined. Form the mixture into meatballs a bit larger than golf balls.
Heat the olive oil in a large non-stick skillet over medium-high heat. Add the meatballs and cook, turning, until browned, about 10 minutes. Transfer the meatballs to a plate.
Discard any oil from the pan. Add the tomato sauce, tomatoes, basil, and oregano to the pan. Stir, and bring to a simmer. Gently place the meatballs in the sauce, cover, and simmer for 30 minutes, or until the meatballs are cooked through. Turn the meatballs about halfway through the cooking time.
Place slices of mozzarella on the meatballs and simmer for an additional 5 minutes, or until melted.