OS X (OS 10) is good enough for Apple,
Wiki says "The letter X in OS X's name refers to the number 10, a Roman numeral. It is therefore correctly pronounced "ten" /ˈtɛn/ in this context."
Windows 10 (Windows X) ought to be good enough for Microsoft.
Last edited by seacue; 1st October 2014 at 11:15.
Reason: Wiki mention added.