Spread flour on plate. Sprinkle oxtails with ¼ tsp of the pepper; press into flour to coat. Reserve remaining flour.
In large Dutch oven, heat oil over medium-high heat; brown oxtails all over, in batches. Transfer to plate.
Drain fat from pan. Add onion, garlic, celery, carrots, oregano and remaining pepper; cook over medium heat, stirring often, until softened, about 5 minutes. Sprinkle with reserved flour; cook, stirring, for 1 minute. Add wine and vinegar; bring to boil, stirring and scraping up brown bits. Add tomatoes, breaking up with potato masher.
Return oxtails and any accumulated juices to pan; bring to simmer. Cover and braise in 325°F oven until meat is fork-tender, about 3 hours. (Make-ahead: Let cool for 30 minutes. Refrigerate, uncovered, in airtight container until cold. Cover and refrigerate for up to 2 days or freeze for up to 2 weeks. Thaw before reheating.