I'm not a big crock pot fan, but actually oxtails are the perfect use for a slow cooker. The whole purpose of an oxtail is to be braised or stewed very slowly, so that all the connective tissue dissolves into this wonderful gelatinous texture (OK, that sounds gross, but it just means that your sauce will be thick and savory and carry flavors well) and the meat becomes meltingly tender. Mmmm, they're SO GOOD! Look up "oxtail stew" or "braised oxtail" in a good cookbook or on Epicurious -- I know that Paula Wolfert's The Slow Mediterranean Kitchen has an oxtail recipe I've always meant to make.
These both sound awesome:
http://www.epicurious.com/recipes/fo...-Oxtails-14520http://www.epicurious.com/recipes/fo...and-Bacon-1503