Pat the oxtail pieces dry with paper towels. Season generously with salt and freshly ground black pepper on all sides.
Heat olive oil in a large skillet or Dutch oven over medium-high heat until shimmering. Carefully add the oxtail in a single layer (work in batches if necessary to avoid overcrowding) and sear for 3-5 minutes per side until deeply browned.
Transfer the seared oxtail pieces to the slow cooker insert.
In a medium bowl, whisk together the dry onion soup mix, Worcestershire sauce, beef broth, and tomato paste until thoroughly combined and smooth.
Pour the prepared sauce mixture evenly over the oxtail in the slow cooker. Ensure the oxtail is mostly submerged.
Cover the slow cooker and cook on the LOW setting for 8 hours, or until the oxtail is incredibly tender and easily falls off the bone. Alternatively, cook on HIGH for 4-5 hours.
Once cooked, carefully remove the tender oxtail from the slow cooker and place it on a serving platter. Skim any excess fat from the surface of the gravy in the slow cooker, then taste the gravy and adjust seasonings if needed.
Serve the fall-off-the-bone oxtail with the rich, savory gravy over your favorite accompaniments like white rice, mashed potatoes, or creamy polenta. Garnish with fresh parsley if desired.
Notes
For an even richer flavor, you can add one bay leaf to the slow cooker with the other ingredients before cooking. Leftovers store wonderfully in an airtight container in the refrigerator for up to 3-4 days, and the flavor often improves overnight. This dish pairs exceptionally well with a simple side of steamed white rice or creamy mashed potatoes to soak up all the delicious gravy.