Heat the vegetable oil in a large pot or Dutch oven over medium-high heat. Add the oxtail pieces and sear them on all sides until browned, about 8-10 minutes. Remove the oxtail and set aside.
In the same pot, add the chopped onion, minced garlic, and sliced ginger. Sauté for about 3-4 minutes until the onion is translucent.
Return the oxtail to the pot. Add the soy sauce, rice vinegar, brown sugar, beef broth, star anise, and black peppercorns. Stir to combine.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 2 to 2.5 hours, or until the oxtail is tender and the meat is falling off the bone. Stir occasionally and add more broth if needed.
Once cooked, remove the oxtail from the pot and set aside. Skim off any excess fat from the surface of the sauce.
Serve the oxtail hot, spooning the sauce over the top. Garnish with chopped green onions.