Now it is neither! As of June 13, 2016, the name has been changed to macOS https://www.apple.com/macos/sierra/
During the keynote at WWDC on June 13, 2016, Apple announced that OS X would be renamed macOS to stylistically match Apple's other operating systems, such as iOS, watchOS, and tvOS.
This is a modified version of my answer on I think system specifications should show as signature?.
I've seen similar systems implemented on other sites (like the Apple Discussions).
The current system is to specify the OS in the body of the question and/or in a tag. I think that way works pretty well; it gives users an easy way to include the info and ...
Different iterations of iOS and Mac OS X have more in common than they have differences. While OS versions are sometimes very relevant to the answer to a program, frequently they are not. I see no reason whatsoever to require questions to be more specific than necessary.
It is not rare for a question to omit important information. Occasionally, that missing ...