Sally Fallon has a good Oxtail soup recipe in her book Nourishing Traditions. She includes barley with hers, but I skip it.
Basically, bake the oxtail for ~1hr. until browned, transfer to a pot. Deglaze the baking pan by bringing some white wine and filtered water to a boil in it (mixing in all those nice juices), and pour the liquid into the pot. Cover the entire oxtail with cold water. Add in some vinegar, thyme, onions, carrots, celery, pepper. Bring to a boil, then reduce to simmer for at least 24 hours. Strain the liquid, pick the meat off the bones and chop finely. Add the meat back into the broth and season to taste.
It sounds like you could do a lot of that in a slow cooker. Good luck with it!