Mix oxtail, water, broth, onion, garlic, onion salt, salt, and pepper together in a large Dutch oven. Bring to a boil. Cover, turn heat down to a simmer, and cook until meat falls from the bone, about 2 ½ hours.
Stir potatoes, carrots, mushrooms, bell pepper, and green onions into the pot. Cover and let simmer until vegetables are tender, 15 to 20 minutes.