What Sylvey said about rejoining! Process is the same as joining the first time.
To elaborate a little bit, it looks like Oberon's pack -- Casa di Cavalieri -- put him on Absentee NPC status
as a courtesy when Oberon went inactive. :) Thus, it can be assumed that Oberon stayed in the pack so you don't have to come up with a reason for him to have left! You are certainly free to discuss this with CdC's leadership if you decide you would rather Oberon have left the pack.