In a large bowl combine the onion, garlic, oats, milk, cheese, egg, worcestershire sauce and seasonings. Mix well then add in the ground beef and mix just until combined. Heat a large skillet with a little olive oil and shape meatballs into small 1" balls. Add to skillet and cook just until browned on all sides. Don't worry if they aren't cooked all the way through they will finish cooking in the slow cooker.
While the meatballs are browning add the soup ingredients EXCEPT for the spaghetti to a large 6 quart slow cooker. Stir well to combine. Add in the meatballs then cover and cook on high for 4-6 hours or low for 8-10 hours. In the last 15-20 minutes of cooking add in the uncooked spaghetti. (I like to break it in half) Serve immediately garnished with additional Parmesan cheese or shredded mozzarella if desired.
This soup will thicken quite a bit when cooled. If you have leftovers and would like it to be more like soup thin it with a little extra broth.