Or does it even matter?
As I mentioned yesterday, I'm editing a ton of question titles. I tend to change OSX to OS X just out of personal preference, but I wonder if that will annoy anyone?
This is now a historical curiosity since macOS replaces OS X. And we have a new data point with the iPhone X in writing being verbalized as iPhone 10 when spoken.
Apple's Mac OS X Lion website, documentation, and marketing material all have spaces between OS and X. I'd assume OS X is the correct way to spell OS X. Sidebar, Mac OS X is pronounced Mac Oh Ess Ten, not Oh Ess Ex. This can be confirmed by using the say command in the terminal:
$ say OS X # Returns Oh Ess Ten
$ say OSX # Returns Oh Ess Ex
$ say X # Returns Ex
$ say Roman numeral X # Returns Ex
Also, as noted by michaelmichael the proper spelling would be Mac OS X.
Now it is neither! As of June 13, 2016, the name has been changed to macOS https://www.apple.com/macos/sierra/
During the keynote at WWDC on June 13, 2016, Apple announced that OS X would be renamed macOS to stylistically match Apple's other operating systems, such as iOS, watchOS, and tvOS.
What is the difference between [mac] and [apple]? (2010-08-17) — AFAICT focused on tagging
On the right name of Apple products and softwares (2010-08-18)