I think part of it is the fact that he didn't actually take the body of the new host, but is sorta sharing part of his spirit, too. Remember when he first talked to his host before he transferred? He said something like he will still be there and be more powerful than ever. Obvious, Ochi dominates the spirit, but the real host may still be there.
Given that, he may be using his spirit to use his arms. Or something.
Also, while Ochi was able to recover from Sarutobi's damage, it still cost him three years before he could transfer to Sasuke. That's valuable time for Kohona to plan before they would rescue him, or at least lick their wounds.
Rosalina: But you didn't.
Robert: But I DON'T.
Rosalina: You sure that's right?
Robert: I was going to HAVE told you they'd come?
Rosalina: No.
Robert: The subjunctive?
Rosalina: That's not the subjunctive.
Robert: I don't think the syntax has been invented yet.
Rosalina: It would have had to have had been.
Robert: Had to have...had...been? That can't be right.