Oxtail is cooked low and slow with red wine until tender and succulent. Peppery watercress is added at the last minute until it's just wilted.
Season oxtail generously with salt and pepper. In a large Dutch oven, heat oil over medium-high. Place flour in a shallow pie pan; season with salt and pepper. Dredge oxtail in flour mixture until coated all over, shaking off excess.
Working in batches, sear oxtail in Dutch oven until well browned on all sides, about 15 minutes. Remove oxtail and set aside. Wipe out Dutch oven and return oxtail to pot. Add wine and enough water to cover oxtail halfway, about 3 cups. Bring to a simmer and cook, partially covered, turning oxtail to prevent drying out. After 3 hours, add additional 2 cups water if necessary. Continue cooking until meat is tender and pulls away from bone, 3 hours more.
Remove oxtail and set aside. Pour cooking liquid into a fat separator. Allow fat to rise to top. Pour defatted cooking liquid back into pot. Cook over medium-high heat until reduced and thickened, about 15 minutes. Return oxtail to pot and stir in watercress. Cook until oxtail is heated through, about 5 minutes.
Serve oxtail either on the bone or with meat removed from bone and transferred to a shallow dish. Pour reduced cooking liquid and watercress mixture over meat and serve immediately.