You didn't mention whether you've been to Korea before but I would infer not. In a similar situation I'd probably take advantage of whatever opportunity there was to see something more of a new country than just the airport. Yes, it's a slow trip on the KAL limo bus in the early evening, but you will see something of Seoul, even if that's nothing more than the traffic, the river, the two dozen or so bridges over it, the COEX IC (one of my favorite hotels in Asia), and the incredible mall underneath it, with scores of places to eat.

By the way, there are two limo bus options: one that stops at half a dozen or so hotels in the Kangnam area, of which the COEX IC is the last, and another that goes direct to the City Air Terminal (KCAT), but entails a walk of 5-to-10 minutes from the terminal to the hotel. And also by the way, you can check in for your departing OZ flight at KCAT next morning.

Thanks for this information. I must try it some time.