Soak the oxtail in cold water for 2 hours to remove the blood. Peel the radish and cut into 5 x 5 cm (2 x 2 inch) cubes, chop the green part of the green onion.
Trim the fat on the oxtail and cook in a pressure cooker with 2 cups (500ml) water for 30 minutes over medium heat. Remove the lid and add 8 cups (2 litres) water and continue to cook for 1 hour. Let it cool and put the oxtail and broth in separate containers in the refrigerator for overnight.
The next day, a thick layer of solid fat will form that you can remove and discard. Combine the oxtail and broth in a large pot and add an additional 6 cups (1.5 litres) water, milk, garlic, radish and the white part of the green onion. Cook for 1 hour over the medium heat and occasionally skim the foam and the fat from the top of the soup. Lower the heat and cook 30 minutes more. Take out the radish and green onion and discard.
Serve sea salt, black pepper and chopped green onion with the soup so it can be seasoned individually.