Oxi was the keeper of rats. He ensured that the rats were well-fed and taken care of, so that they were in peak condition when they fought the overgrown cats that often visited the pits. He also ensured that they did not escape and infest the rest of the cave, although they did break out from time to time. He didn't find this to be much of a problem, though, as there were plenty of cats in the area.
Oxi carried a large bag on his back, filled with supplies needed to keep the rats alive. Prior to the dwarves graphical update, he also carried a walking stick, although this may just have been to help remove the heavy burden of the sack.