Pauli Olavi Ojala, writing for Medium:

The Mac operating system holds the dubious honor of being the most frequently mispronounced tech brand. It looks like “Oh Ess Ex”, but the X is actually a roman numeral, so you’re supposed to say “Oh Ess Ten”.

And:

All of Apple’s other operating systems are branded with a lowercase prefix and a simple version number. Meanwhile OS X has a total of three numbers in the name: there’s the roman “X”, then the same number as the major version number, then the actual version number which is currently eleven.

There’s a simple solution: bring back the “Mac” and drop the confusing non-version-numbers.

At the very least, there’s a lack of branding cohesion here. The Mac has a long tradition with Roman numerals, starting with the Mac II and shifting to the OS when Steve Jobs came back to Apple and used the seeds of NeXTSTEP to create OS X.