Preheat a large skillet over medium-high heat. Season the oxtails with salt and pepper. Add a drizzle of olive oil to the skillet and brown the oxtails on all sides, about 2-3 minutes per side.
Once the oxtails are browned, transfer them to the crock pot.
Add the onion, garlic, carrots, and celery to the same skillet and sauté for about 5 minutes, or until they start to soften.
Pour in the beef broth and red wine, and scrape up any browned bits from the bottom of the skillet. Add the diced tomatoes and bay leaf.
Bring the mixture to a simmer, then pour it over the oxtails in the crock pot.
Cover the crock pot and cook on low for 8-10 hours, or on high for 4-6 hours, until the oxtails are tender and falling off the bone.
Once the oxtails are done, remove them from the crock pot and transfer the sauce to a saucepan. Simmer the sauce for 10-15 minutes, or until it has thickened slightly.
Serve the oxtails with the sauce on top, and garnish with fresh herbs or chopped parsley, if desired.