Pat the oxtail dry thoroughly before seasoning with salt, pepper, and flour because removing excess moisture helps the meat brown properly instead of steaming in the skillet. Brown each piece in olive oil until deeply golden for the best flavor foundation.
Cook the onion, carrots, and celery in the same skillet until softened, then stir in garlic and tomato paste. Allow the tomato paste to darken slightly because this develops a deeper savory flavor in the final gravy.
Transfer the vegetables to the crockpot and add beef broth, crushed tomatoes, Worcestershire sauce, smoked paprika, thyme, oregano, and bay leaves. Stir gently to combine the braising liquid evenly.
Add the browned oxtail into the crockpot, cover, and cook on low for 8–9 hours or until the meat becomes fork tender and the connective tissue softens completely.
Remove the cooked oxtail carefully, then stir cornstarch slurry into the sauce and cook briefly on high until thickened. Return the meat to the crockpot and spoon the gravy over the top before serving.