In a large pot or Dutch oven, heat the olive oil over medium heat.
Add the oxtail and brown on all sides.
Remove the oxtail from the pot and set aside.
Add the onion, carrots, celery, and garlic to the pot and cook until softened, about 5 minutes.
Stir in the thyme, bay leaf, salt, and pepper.
Add the beef broth and red wine (if using) to the pot.
Bring the soup to a boil, then reduce heat to low and simmer for 2 hours, or until the oxtail is fall-apart tender.
Remove the oxtail from the pot and shred the meat with two forks.
Return the shredded oxtail to the pot and stir in the flour until it is dissolved.
Simmer the soup for 10 minutes more, or until it has thickened slightly.
Garnish with chopped fresh parsley and serve hot.