As I was trawling through the archive of the Daily Express, I stumbled on an interesting article from the edition dated 14th September 1966. It was interesting not so much for its content – alas I didn't make a note of what it was about – but rather its headline: “Little Mrs 006 does an 007.” The use of the indefinite article 'an' suggests that the headline writer intended '007' to be read as 'O-O-Seven' (that is, Oh-Oh-Seven), as opposed to 'Double-O-Seven'.
Although Ian Fleming was clear on how Bond's code number should be pronounced – in Casino Royale (1953), Head of Section S thinks the Le Chiffre job will go to “one of the Double-Os” (Chapter 3), and in From Russia, with Love (1957), the Soviet dossier on Bond mentions Bond's “double 0 numerals” (Chapter 6) – the headline suggests that the alternative O-O-Seven had a degree of penetration in popular culture.
The use of O-O-Seven was not confined to the Express. For example, the Bond parody Loxfinger (1965) by Sol Weinstein gives the protagonist Israel Bond the code name 'Oy-Oy-7', and in the Bond spoof, Carry On Spying (1964), agent Charlie Bind (played by Charles Hawtrey) explains that his code number, Double-O-Oh, comes from his instructors looking at him and saying, “O, O, Oh!” A somewhat egregious use of O-O-Seven is found in the film From Russia With Love (1963). Robert Shaw's Red Grant tells Bond to “take it easy, O-O-Seven.” It could be argued, of course, that this is a deliberate mistake offering Bond a clue that Grant is an enemy agent, but unlike the red wine with fish, Bond never picks Grant up on it. In any case, simply by being uttered in an official Bond film, the use of 'O-O-Seven' gains some validity and prominence.
While 'O-O-Seven' has never seriously competed with 'Double-O-Seven', it nevertheless exists as an alternative form of Bond's code name and meme in its own right. Indeed it continues to be used; the details escape me, but I recall the BBC newsreader, Philip Hayton, who presented the news on the BBC between 1987 and 2005, announcing the new 'O-O-Seven' (probably Pierce Brosnan). Its use may be attributed to a lack of familiarity with the books or the films, but even with regular exposure to the correct form, 'O-O-Seven', once established in individual minds, has a good chance of being replicated simply through force of habit.