By 6:30p, it shouldn't be as heavy for transit. For KE and OZ (largest tenants aka most traffic) the bulk of their America/Europe flights arrive 3-5p. Their evening flights to SE Asia start around 4-6, with some "late" ones around 8. You'll be fine. If you do find yourself in a massive line, show your itinerary to someone.

Generally, if you are cutting in close, you can skip security lines AT start of boarding time, which with Asiana, should be 30-35 minutes before departure. Show your boarding pass/itinerary to an ICN staffer managing the line. This is also true landside after check-in. You'll be fine!