Dust the oxtail with flour and season with salt and pepper.
Heat oil in a large pot and brown the oxtail on all sides. Remove and set aside.
In the same pot, sauté onions, carrots, celery, and garlic until fragrant.
Add tomato paste and stir for 2 minutes.
Return oxtail to the pot and pour in red wine. Simmer for 5 minutes.
Add beef stock, bay leaves, and thyme. Bring to a boil, then reduce heat and cover.
Simmer for 3–4 hours, stirring occasionally, until meat is tender and falling off the bone.
Add potatoes in the last 30 minutes if desired.
Remove bay leaves and adjust seasoning before serving.