I prefer "Oh Es Ex" myself, but as I've mentioned here before, since the X can be construed rightfully as a Roman numeral 10, it serves a double function--in essence, it's a pun or non-sexual double-entendre of sorts.
Of course, we'll find out for sure once "System 11" comes out.
Note too that at Apple's site, there are references to, and I quote, "
Mac OS X v10.4 Tiger" and "
Mac OS X version 10.4 'Tiger'." (The latter link is a PDF file; the reference comes specifically from page 3 of the Tiger product guide.)