I seem to remember Jobs up on stage announcing "Oh Es Ten".
Just watch the latest keynote: around min 21 ...
At 2:05 Tim Cook calls it O S Ten. So that would be the guideline of how Apple pronounces it.
YouTube: video
As I said in my original post, the question is not asking how to correctly pronounce OS X; neither is the question asking how Steve Jobs or Tim Cook pronounced OS X. The point of the question is to find out how ordinary Mac users pronounce the word.