In a large bowl or resealable plastic bag, whisk together the Worcestershire sauce, soy sauce, brown sugar, onion powder, garlic powder, thyme, smoked paprika, and optional cayenne pepper. Add the oxtails and toss until they are evenly coated. Refrigerate and let marinate for at least 3 hours, or overnight for more flavor.
Remove the oxtails from the refrigerator about 30 minutes before cooking to allow them to come to room temperature.
Heat the olive oil in a large, heavy skillet or Dutch oven over medium-high heat. Working in batches to avoid crowding the pan, sear the oxtails for 2-3 minutes per side, until deeply browned.
Transfer the seared oxtails to the basin of a 6-quart or larger slow cooker. Pour in any drippings left in the skillet.
In a medium bowl, whisk together the beef broth, condensed cream of onion soup, and the dry onion soup mix until smooth.
Pour the broth mixture over the oxtails in the slow cooker. Stir gently to combine.
Cover and cook on LOW for 6 to 8 hours or on HIGH for 4 to 5 hours. The oxtails are done when the meat is exceptionally tender and pulls away from the bone easily.
Check that the meat has reached a safe internal temperature of at least 145°F (63°C). Let the meat rest for 3 minutes before serving. Skim any excess fat from the surface of the gravy if desired, then serve the oxtails with the gravy spooned over top.