It seems that you are saying that you can book to Busan, just not at a cheap price. I'm guessing this is because, apart from the LH flights a few times a week, there are no direct flights. This makes competition poor and will limit thru prices. Oneworld is really about convenience, not price, even then, I think the benefits are oversold.

Have you tried an all JL itinerary? It may cheaper, sticking with one carrier normally is. I used to travel to USN and had to settle for separate tickets with a ICN to GMP connection.