If you use the say command in the Terminal you get the definitive answer.

Spoiler alert for anybody without access to a mac right now: $ say "OS X" pronounces it as "OS Ten".

Mine says "os ex". (10.11.5)

There has to be a space between the OS and X for it to work for me.

All caps.

Its rules for pronouncing "X" are also distinct in these cases:

  say "Malcolm X"
  say "Louis X"

Hah, it does roman numerals up to 49

say Louis XLIX = Louis the 49th say Louis L = Louis L

That's true for macOS Sierra, but will Mac OS X 10.11 El Capitan and earlier be retroactively rebranded as macOS? Now we will have to listen to people say things like "I upgraded my Mac from Mac OS X 10.10 to macOS Siera". :)

It was called "OS X", not "Mac OS X" since 10.8.

Minor correction. The switch from "Mac OS X" to "OS X" happened with OS X 10.7 Lion, not with OS X 10.8 Mountain Lion. The last release with the "Mac OS X" name was "Mac OS X 10.6 Snow Leopard".

I think Lion was released as "Mac OS X", and they changed it to "OS X" later.

http://www.macrumors.com/2012/02/16/apple-officially-drops-m...

http://appleinsider.com/articles/12/02/16/with_mountain_lion...

Thanks! I forgot about that name change. We've come full circle:

Mac OS 9 -> Mac OS X 10.0 -> OS X 10.8 -> macOS 10.12

Is it Mac Oh Ess or does it rhyme with GladOS?

Mac Oh Ess, like iOS, tvOS, and watchOS.