I've bought my Asiana ticket on Orbitz many times. Just going to share what I've noticed over the years. On Orbitz's website you will need to enter your Asiana club membership number when purchasing the ticket or it might not show up on Asiana's site when you log in. If you have the ticket number starting in 988 you can try calling Asiana to have the membership number linked to that ticket. Also, it takes several hours after purchasing the ticket on Orbitz for the record locator to show up on Asiana's website. So if you try to go to Asiana's website to assign a seat right after purchasing on Orbitz, the Asiana website will say there is no such ticket.

As for seat assignment, Orbitz usually takes your preference (seat or aisle) if you selected on and that basically gets auto assigned. I think it just selects the first window or aisle available.

What is interesting is that Asiana's website was updated very recently. A lot of features were added but unfortunately it still leaves a lot to be desired.

I agree with chocobill, calling Asiana will usually resolve any issues with seat assignment.