I know we're supposed to pronounce OS X as "O-S-Ten", but I refuse. The official pronunciation was an old marketing gambit by Apple, an attempt to convince customers that OS X was the next incremental improvement on Mac OS System 9. It worked for the most part, but years have since passed, and it's time to give up the pretense that OS X was an incremental update to System 9.
"OS X" was actually a pretty clever name for Apple's once-new OS, not just because of the sequential serendipity of the Roman numeral 10, but because X was also a reference (to those in the know) to the unix at the heart of the system.
For reasons that have to do with 1980s-era trademark disputes that are so complicated that they are still being fought about today, most companies producing operating systems based on unix technology forsook the Unix brand, and adopted another name with an "X" in it. This is how the world ended up with not just SysV Unix, but also AIX, A/UX (Apple's own), Ultrix, Irix, Xenix, HP-UX, Linux, and many others. The attempt to standardize methods across all of these systems was itself called POSIX. The cross-platform unix windowing system was simply 'X' (although, like OS X, there is a double-pun here since it's non-unix predecessor was called W). This naming scheme was a subtle and trademark-free form of co-branding, and few unix companies elected to forego this little trick. (Sun was one of the few to succeed with SunOS/Solaris.)
NeXT was another in the long line of X-brand unix OSes, and in the unix tradition made liberal use of puns in its obscure nomenclature. Aside from the 'NeXT' name itself, the system kernel was called XNU. This is just UNIX spelled backwards. Well, almost: XINU was already taken, but XNU was more or less the same acronym: X(i)NU is Not Unix.
Anyway, XNU is still the kernel of OS X, which is basically a rebranded NeXTstep, so it's pretty clear to the hard-core nerds what the X in OS X really refers to here. So some of us have long ignored the official pronunciation guide and deliberately said "O-S-Ex" even after being corrected by people who read too much Apple PR.
Apple's attempt to rewrite its own history has always had one really clumsy side-effect. What to call new releases of "O-S-Ten"? The obvious choice would seem to be "O-S-Eleven" etc., but clearly Apple has avoided this, giving the lie to the whole "System 10" myth. They really can't decide whether X is a version number or an X-brand unix reference. In a misdirected attempt to clarify things, they explicitly added version numbers 10.* after the X. So we're supposed to say "O-S-ten version ten? For a company that prides itself on good design, this is surely one of the goofiest marketing decisions ever. It's not only redundant, but it's also redundant. Will there ever be an "O-S-ten version eleven"? Or should we expect one day, not too far from now, an "O-S-ten version ten point ten"? Did I mention redundant?
Plus it leads to pointless confusion about paying for minor-version-number upgrades every time a new release of OS X comes out. Apple's even goofier way of addressing this is not to adopt a sensible version numbering scheme, but to name the versions after cats. But I for one can never remember whether panther beats tiger, or if it's the other way around.
Clearly one part of the marketing department was enamoured of the false "System 10" mythology, while another part of the marketing department thinks OS X can stand on its own two feet as an X-brand unix without a nod to obsolete product lines that nobody cares about any more. But the series of compromises that these two groups have come up with to support their different visions of the OS X brand are just plain dumb, and they get dumber with every year that System 9 has been dead and buried. Can we let it go already?