Soak the oxtail in cold water for 30 minutes, then rinse well.
Place the oxtail in a pot, cover with water, and bring to a boil. Discard the water and rinse the oxtail clean.
Return the oxtail to the pot. Add fresh water, onion, and garlic.
Bring to a boil, then reduce to a gentle simmer and cook for 4 hours. Skim off the oil, aromatics, and impurities.
Add the daikon radish and continue simmering for 40 minutes, or until tender.
Garnish with chopped green onions.
