O.P. is the description of a forward step when taken outside partner. P.O. is the description of a back step when a partner is stepping outside of you. This is true of man or lady.
Most technique books do not make the distinction. They describe O.P. steps, but when you are the person stepping back, they do not indicate anything (other than CBMP, which almost always accompanies OP/PO). I guess they figure the honus is on the person moving forward to position themselves in such a way as to move outside partner.
This may well be true, but we feel nonetheless that it is useful information to have, even for the person moving backwards. It is especially imporant information for the man to have when moving back, since he's supposed to be leading the lady to step outside of him. Therefore, we have decided to indicate P.O. for the person moving backwards as a general rule. It's not the norm, but I think it should be.
Sincerely,
Jonathan Atkinson
www.ballroomdancers.com