Soak oxtail in cold water to remove blood, about 2 hours. Drain. Trim fat off the oxtail.
Place oxtail in a pressure cooker; add 2 cups water. Cover and cook over medium heat according to manufacturer's instructions for 30 minutes. Add 8 cups water and cook for 1 hour more. Remove from heat; release pressure according to manufacturer's instructions. Let cool, about 30 minutes.
Transfer oxtail to a container using a slotted spoon. Pour broth into a separate container. Cover and refrigerate, 8 hours to overnight.
Discard layer of fat that formed over the surface of the oxtail and broth.
Combine oxtail and broth in a large pot. Add remaining 6 cups water, daikon, white parts of the green onions, milk, and garlic. Cook over medium heat, skimming foam occasionally off the top, until garlic is soft, about 1 hour. Reduce heat to low; cook for 30 minutes more.
Remove daikon and white parts of the greens from the soup with a slotted spoon; discard. Garnish soup with green parts of the green onions. Season individual bowls with 1 teaspoon salt and 1 teaspoon black pepper.