You've got a couple of options:

1) You should be able to find either your OZ PNR somewhere in your receipt from TK and you can use either the PNR or your ticket number to find you OZ flight from ICN on the OZ website and choose your seat there.
2) You can call OZ and they should be able assign seats for you. Sometimes you have to call anyway because of IT issues.

Also depending on when your flight leaves you might be locked out from seat selection T-2hrs since on line check in is closed and on US flights from T-48 to T-24 because the OZ system closes seat selection at T-48 and sends it to the online check in system but you can't check in for US flights until T-24 so all seat selection is blocked.

Hope this helps and enjoy your trip.