CJ,
Old line Agents will tell you (as will early rule books) that in reading back Train Orders to the dispatcher, the Number zero was to be pronounced "Naught". We still have some older conductors who use this form when verbally communicating via radio. I got this from an old C&S Agent and he told me that this is the way that he was taught in school. Yes, there used to be independent schools to teach prospective Railroad employees the proper way of doing their jobs, in the Agent's case, including Typing, telegraphy, train orders and billing.
So, according to (real) Railroad Parlance, the D&RGW MOW equipment OA-OZ would be pronounced "Oh" as in the letter between N and P. The cabooses in the trains would have their first number pronounced "Naught".
Seems simple enough...
Rick