Is the flight chronically late departing ICN, or just departing ROR? The flight is scheduled to depart ICN at 23:10. I fly out of ICN a lot, and usually on OZ, but never had a flight at that hour. However I don't see why air traffic would be a problem at that time. ICN is not a 24-hour airport with regard to passenger flights. I imagine there's a number of cargo flights at night, but passenger flights are extremely sparse after 22:00. I have experienced frequent delays earlier in the evening due to congestion, but that is always only a matter of minutes, not hours.
More likely for a long delay would be if the aircraft used for that flight is late arriving from a different location. I've had that happen fairly frequently, but again never anything like 4 or 5 hours. An hour or so would be the maximum.
So if this flight is chronically late, I'd say it's probably due to something at ROR, not ICN. Perhaps ROR for some reason doesn't want the plane to arrive and/or depart at it's scheduled time. Seems a bit strange that whatever the reason, OZ doesn't just reschedule the flight though, rather than allow it to be late so often.