There is no MacOS. There is no MacOSX, nor MacOS9, nor OSX.
There are two operating systems, Mac OS, and Mac OS X. The former is currently at version 9.2.2, and the latter at 10.1.5. The full names are "Mac OS 9.2.2" and "Mac OS X 10.1.5". Throw in a "v" or "version" if you like.
Using "OSX" is barely acceptable, if only because so many people use it, but it is too non-specific: what is "X"? If "OSX" is to be used for "Mac OS X", then why not "OS" to refer to "Mac OS"? It should be avoided.
"MacOSX" is not acceptable, nor any other form omitting the space between "Mac" and "OS". Using "Mac OS" to mean "Mac OS X" is not acceptable; they are two different operating systems. "Mac OS 9" is acceptable, to distinguish it from previous versions of Mac OS, and barely acceptable to distinguish it from "Mac OS X", only because so many people incorrectly use "Mac OS" to refer to "Mac OS X".
Also printed on my personal site.