I don't know about your ticket, but my OZ ticket provided for either a transit tour or a hotel (outside the airport). I had an 8 hour layover. The transit tour included a stop outside the Presidential palace, some mall, and a traditional Korean lunch. It was about a 45-min ride from ICN to Seoul.

I know there is a transit hotel in the terminal you can use by the hour, but with that much time I would find a tour into Seoul for a look-see.