Begin by heating the oil in a cast-iron pan. Season your oxtail pieces and then brown them in the pan (about seven or eight minutes of cooking time total).
Then, transfer the browned pieces to a bowl and pour off any remaining fat.
Add the chopped ginger and garlic to the pan to cook until golden (about five minutes). Stir in your sake and scrape up any remaining browned bits from the bottom of the pan. Add your beef broth and simmer, then return the oxtails.
Transfer everything to a large stock pot and cook for about three hours, or until the meat is tender.
If you want a broth that contains less fat, after the three-hour time period has elapsed you should transfer the oxtails to a baking sheet, remove all the meat from the bones, and discard all the bones, fat, and gristle. You should also strain the broth, skimming off any remaining fat. This is not a mandatory step, but does make for a cleaner, lighter soup.
Return the broth to the pot, adding the daikon and leeks. Cook for another forty-five minutes.
While the vegetables and broth are finishing up, cook the ramen. Bring a pot of water to a boil, then add the ramen, stirring often. It should take about five minutes.
Rinse and drain the ramen and place into the oxtail soup to absorb the flavors