Yeah that was never adequately explained which still annoys me to this day. If Oro's soul is what he transfers into new bodies and having part of his soul missing caused physical damage to manifest on the body he was currently in it doesn't make sense that transferring the remainder of his soul into a new body would fix his hands.
The best I can come up with is that he gained the use of the hands of the new body because his soul merged with the soul of that body which hadn't had it's hands sealed so he sort of inherited that bodies 'soul hands', this makes enough sense for me, but I would much rather it had been explained by KIshi.