I'm pretty sure the Micropad OCR gadget (circa 1980) that I once had required
 slashing of the letter 'o', with the unslashed version interpreted as a zero.
 (and to comment on another thread, I think the 286 Compaq luggable that I had
 for a while had a dot in the middle of the zero rather than a slash - I'm not
 sure if that was a standard feature or something oddball about my machine) 
FWIW, I've just pulled a book off my bookshelf. 'Assembler Language
Programmign, The IBM System/360 and 370', 2nd edition 1975.
It says  :
'Note that the letter O (Oh) is written as 0 to distinguish it clearly
 from the digit O (sero). We follow this convention in
the description of  
any statement or card that is fed to the computer'
So in that book (and presumably in other related manuals/documentation),
the letter gets the slash.
-tony