How to Debone Oxtail?
Let the butcher do it
This is roughly the bone structure of a mammal tail:

As you can see, it has very many small bones, with of course all the connective tissue. Unless you have solid knife skill, a very sharp boning knife (with the needle-like blade), a protective chain mail glove, and lots of patience, I would say something like this is better left for a professional butcher. Especially if you have not done much boning, this could easily end up in an injury.
For reference
Boning knives:

Chainmain butcher glove: