Your memory is incorrect. The branding sequence was System (from before I remember up to 7.5) -> Mac OS (7.6 to 9) -> Mac OS X (10.0 to ??) -> OS X (??) -> macOS (starting after 2015).
Edit: Sibling comment from htimsenyawed looks right to me.
Re:Rust on macOS 9
by htimsenyawed on Sunday January 22, 2023 @09:38PM
(#34484796)
macOS was originally Mac OS X and short referenced as OS X. The official name change from Mac OS X to macOS began with release 10.12 Sierra, the previous version was called Mac OS X 10.11 El Capitan. The transition to macOS from Mac OS X occurred in 2016 as that was the year that Sierra was announced and released.
I’d just like to interject for a moment. What you’re referring to as OS X, is in fact, Mac OS X, or as I’ve recently taken to calling it, macOS.
Also, and I'm not sure if I remember that correctly, I think during "Mac OS X" times they said it is pronounced as "mac OS ten", because X is the roman numeral for 10.
Wikipedia has references about it where Apple themselves even back then used either pronunciation, though. Also a reference that says while it stands for 10 it also references the X commonly found in many Unix things because of the Unix roots of Mac OS X.
At the time I found it mildly frustrating because of the stupid casing on top of the pointlessly annoying pedantic distinction destined to cause confusion. As if there were another "OS-X" of any significance.