I say "OS Ex". I like the way it sounds better, and I think that Apple had to know people would call it that. In fact, I think that most people say "OS Ex". And, yes, I fully realize that the correct pronunciation is "OS ten". However, 99%+ of the time, I'm writing it, not saying it, so it doesn't matter. In fact, when I talk, I almost always use "Panther", "Jaguar", etc. So, in the end, I'm not even sure I've actually verbalized it as "OS Ex" to anyone. However, in the voice in my head, as I read it, it's "OS Ex". Since I always wear my aluminum foil pointy cap, my brain waves aren't influencing anyone else, so I suppose it's my business alone how I choose to pronounce it internally.
I also say - out loud - "jiff" instead of "giff" when I'm talking about GIF files. Anyone have a problem with that? 😉