The Arex express is 43 minutes from Seoul Station and, at certain times of the day, can be a good choice. Also, you can check bags for OZ and apparently go through immigration as well (at least that's what the signs say).
There are some F lounges that I would definitely get there early for, the OZ F lounge is not one of them. If you try to get there at the 2hr mark you'll be doing fine.