Oxtails are awesome, but not sure I would smoke them, although there is no reason you can't.
It's just that they lend themselves to braising as SCUBADOO97 said, and should be used in stews and such, since there is little meat on them.
Smoking them for a bit would add flavor, then you can transfer them to a recipe where they are braised.
Just be aware they are tough, boney and fatty. The bone, marrow and tissue is what makes them perfect for long, liquid cooking.
Seems like a pointless excercise to smoke them, but that is merely my opionion. They are excellent dredged in flour, browned in oil and put to the "braise"
If you do smoke them, please let us know how they turned out. You might have another fantastic idea!