>Why not just deal directly with your local OneWorld ticketing office?

Finding a good TA is as difficult as finding a good doctor or lawyer, and I don't generally defend them as a group (and won't this time) but there can be significant benefits to using one to book an OWE. Most important to me is that they're your personal contact who immediately knows you and what you need when you call. The AA exp's used to crow about getting that kind of service from their explat desk, but the rest of us get potluck when we call an airline.

If you're not used to dealing with a TA, remember that some airlines give them zero or near-zero commissions, while others are somewhat fairer.