I've been looking for this, and can't find anything specific.
However, jws do have a penchant for shortening the name of their
publications, ie the "live forever" book, the "knowledge book" etc
were both shortened titles of books that were used in the field
ministry for studying with interested people, and became known
in jw circles by those shortened titles.
I looked through a list of wts publications, and found this
Organization for Kingdom-Preaching and Disciple-Making (1972
O.P. might well be how this book was known when it was being used.
Having not been a jw then, I have no way of knowing, so perhaps
someone who was in the org then may know.