Yes, you can call OZ and ask them to book the flight; in MOST (but not all) cases, the first character of the fare code is also the booking class. So you would do well to tell the OZ agent that she should book you in that particular class. After the reservation is complete, ask her to price it. If the price is what you expect it to be, you are home and dry. If higher, tell her that you believe fare code XYZ should price at $ABC, so why does it not in this case and what can you do to get the lower fare?

There are hundreds of factors which may cause a variance, but you should let them work it out. Good luck, since the OZ agents are not the brightest!