Personally I always just call OZ and buy my tickets from them. Though recently I haven't been purchasing tickets ex-USA. One time the OZ agent at LAX told me that I should check with a travel agent in Koreatown next time because they have lower prices. I never did though being I'm always in a rush and it's just easier for me to book via the phone with OZ and then pay for my ticket at the airport when I check in.

What I do is find what's the lowest fare booking class that I'm willing to buy, then I go onto expertflyer (need to pay money to join) and search for the dates around the times I'm interested. Then after finding the dates I want with the cheapest fare booking class, I call up OZ and reserve my ticket. Being D+, there's no ticketing deadline so I simply purchase the ticket when checking in. For non D+ members, if there's a ticketing deadline, then you'll need to purchase before that. I've never purchased over the phone for an ex-USA ticket so don't know if there's any restrictions or not. I have for ex-ICN and ex-BKK/HKT before with no problems, using a Korean credit card. I assume it would be the same for ex-USA flights.