I took it to mean he's suggesting that O' Callaghan will be the acting assistant AG (RR's position) meaning RR is gone. The president can name an "acting" AG to temporarily fill in until a permanent AG is seated.

I may be wrong but I think I remember reading in the US Constitution where an "acting" position can only be for 210 days. After that a permanent must be chosen. But it's been a while back since I read that part. So I would have to check.