So, what are you talking about? Pre-OSX?
Ancient Apple computers ran MacOS, with the same name as the modern OS but with different capitalisation, and he's apparently referring to that. Considering people not part of Apple use all sorts of capitalisation schemes for the modern OS and the subject is obviously the modern variety of the software, I'm not sure posting irrelevant information which might confuse uninitiated passer-bys is considered funny.
There's no accounting for taste, I guess.
I ask because my first computer was a Mac 512 KE when I was 8 and I had two pre-OSX Macs after that. I spent my formidable years on pre-OSX Macs. The Mac OS, prior to OS X was always referred to as "System xx", where the xx was the version number. Even OSX would have followed this originally, as it was technically "System 10" (the X being 10), although I guess Jobs wanted a more modern naming scheme.
I have never heard of pre-OSX being referred to as, "MacOS".
I guess you were no longer using the Mac during the mid- and late 1990s? It was officially the "Mac System Software" ("System" for short)* up through System 7.5.5, but the first appearance of the "Mac OS" name was the boot splash screen in the 7.5.1 update in 1995, in preparation of the Mac clones.
As of 7.6, the Mac OS branding became official, and that naming was used consistently throughout both marketing materials as well as in the software itself. The change roughly coincided with the launch of the Mac clones in 1996, where naming was needed that branded the OS even when running on non-Apple hardware.
As a reminder, Steve Jobs didn't return to Apple until 1997.
Mac OS X would never have been "System 10" because the "system" naming had been retired for many years by that point. (Mac OS X 10.0 was released in 2001, though "Mac OS X Server 1.0", which was really just the final version of OpenStep, dolled up with the skin of Mac OS 8, was released in 1999, whose only real use was to be a NetBoot server for iMacs in enterprise environments. For everything else at the time, an AppleShare IP server was more useful.)
For sure, there were holdouts among users, who insisted on referring to Mac OS 9 as "System 9", but that was never officially correct.
Fair enough. I just like the way OS/X looks better. Not to mention I was focusing on capitalization, not spelling/spacing.
For sure you're allowed your preference, but I don't actually recommend using it in any text intended for anyone else. It's just plain clearer to use the official spelling consistently, lest the reader incorrectly infer a difference. (Can you tell that I worked as a professional technical writer for years?
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(And yes, I think it was unwise of Apple to change Mac OS X's "official" name twice, it's confusing to people who only follow Apple products casually. Though at least the second official name, "OS X"-without-the-Mac, was what lots of people called it casually anyway. macOS is consistent in their modern naming scheme, but I don't think anyone would have felt bothered if Apple's OS lineup had remained "Mac OS X", iOS, tvOS, and watchOS. For completeness' sake, I'll mention that the "old" Apple TV products, the ones without Siri, ran software simply called "Apple TV Software x.x", with versions 1-3 being based on Mac OS X 10.4 for the original Pentium-based Apple TV, while versions 4-7 for the ARM-based Apple TV 2 and 3 were based on various versions of iOS.)
*On early versions of the Mac System Software, the version numbers of the "Mac System Software" as a whole (as labeled on the boot floppy), of the System file, and of the Finder were all separate, which was very confusing. Starting in System 6, the major version numbers were finally harmonized and stayed in sync, though differences in the minor versions did exist. See
Macintosh: System Software Version History