I always say Oh Ess Ex, but maybe I'm weird.

Ten ten. Alternatively Yosemite. Or OS ten Yosemite.

No one will use ten ten ten.

OS X (previously Mac OS X) is the successor of Mac OS 9. X stands for 10 in roman numerals: http://en.wikipedia.org/wiki/Roman_numerals

For marketing reasons (as some don't know about roman numerals), Apple writes it like this OS X 10.9 or now OS X 10.10 or OS X Yosemite (instead of Mac OS X.10 or Mac OS 10.10).

You wouldn't call it Windows Eight Eight Point One, would you?

I think printing it as "OS X 10.10" is just a shorthand for "OS X (10.10)", which is necessary since combining roman numerals and digits (i.e. 'OS X.10') is awkward. You would still say it as "Oh ess ten point ten."

Oh Ess Ex Ten Point Ten?

OS<insert censor blur here>XXX

They already dropped the first ten. It's OS 10.10.