Ok, let me start out by saying this is going to be a page filled with supposition, guess work and, above all, plenty of cynicism. Don't believe anything you read here, it is only the biased and cynical opinion of a warped and twisted mind.
So... Why do I think Apple named their operating system OS X?
Well, I believe it all started with Microsoft. Yes. It's because of Apple's biggest rivals, the Behemoth of Redmond.
Microsoft have long had a fixation on the letter X. Quite why, I have no idea. But if you look at the history of their products and services you will find it littered with the letter X. The XBox. DirectX. ActiveX. It's X's all the way down, as it were.
No doubt when Windows 10 came out Microsoft really wanted to call it "Windows X", but there's two good reasons (well, one good reason and one very silly reason) why they couldn't:
There's been X Windows since the dawn of graphical user interfaces. Windows X is just too close to that to be usable as a trademark, and
It sounds too much like "Window sex", which is something you see in the red-light district of Amsterdam.
So they really couldn't.
But I digress. Back to apple.
When they progressed from System 9 to the next version, what would have been System 10, they thought to themselves: "Hmm.. 10. Roman numeral X. We can steal the X from Microsoft. That'll really annoy them."
So they did. They raided Microsoft HQ in the dead of night and stole their X. And now Apple are the ones with the "funky X" to use for their operating system.
But there was one more glitch. Up until this point Apple have been calling their operating systems "System 7", "System 8" etc. So why isn't System 10 called "System X"? Well, no doubt they wanted it to be called that, but unfortunately for them that name was already taken by the UK's Post Office for their telephone exchange digital switching system back in the '80s. Maybe "Operating System X" was considered and discarded as being too long in favour of a simpler "OS X".
But now that OS X version 11 is out why is it still OS X? Surely now it should progress to OS XI? Well, maybe that just sounds too Chinese, and America doesn't like China at the moment according to the media. Besides, they seem to have abandoned the X after all the trouble they went to stealing it from Micosoft. I wish they'd make their mind up about their brand.