Originally Posted by jjpb3
(But I've heard of cases where the op-up was at the gate.)
I think there are more op-up at the gate rather then check-in. The reason is that check-in agent does not know how many "no-shows" will be on the particular flight. So, after the check-in cut-off time, they can then know how many pax travelling on that flight, and they can then know how many "overbooked" seats on each class. Then, they upgrade people at the gate...