A little history may bring clarity...
Guys,
I think there is a split here between the old-time Mac guys and gals (from OS9 and before) and the Unix guys and gals (who joined the Mac community since OS X)...
The mac guys see OS X as OS 10 because it (spiritually, if not temporally) came after OS 9.
The Unix guys see OS X as just another in the LONG line of Unixes, all of which end in X (which is pronounced "ex"). For example:
From AT&T: UNIX
From HP: HP/UX
From IBM: AIX
From Linus Torvalds: Linux
From Apple: A/UX
See the pattern...?
I am in both old mac and Unix camps, but I've spent much, much longer in Unix than in OS9 or previous, (and I also used OS/2 from IBM) so I pronounce it "OSS EX". Except in Apple stores where I can't be bothered with an argument
It seems that Apple deliberately chose the suffix X to highlight the new OS's Unix ancestry with its stability, security, multitasking abilities etc. and to invite unattractive comparisons with Windows. Which isn't actually fair on Windows, because NT had VMS ancestry which was much more like the Mac ethos than Unix was (just as stable as Unix, but far more friendly, from a single company provider of both hardware and software).
Let's all chill out and accept that some people think different.
Cheers
YS