1. Preheat oven to 350 degrees Fahrenheit. Heat oil in a large Dutch oven or ovenproof pot over medium-high heat.
2. Sprinkle oxtails with salt and pepper, then flour. Add oxtails to pan; cook until brown on all sides, about 15 minutes total.
3. Transfer to a bowl. Add celery, carrots, and onion to pot. Reduce heat to medium-high and sauté until vegetables brown, about 15 minutes.
4. Add wine and tomatoes. Boil until mixture thickens to a chunky sauce, about five minutes. Add garlic.
5. Tie parsley, rosemary, and bay leaves with kitchen string and add to pot. Return oxtails to pot. Add broth and bring to a boil. Cover pot and transfer to oven.
6. Braise oxtails until very tender, about two hours. Transfer oxtails to rimmed baking sheet.
7. Using potato masher, crush vegetables and meat in the pot to coarse sauce. Pull meat off oxtails and add to sauce. Season with salt and pepper to taste. Toss ragu with potato gnocchi and serve.