I always thought it was pretty clear to say "os ecks" myself, but I've heard internally / correctly people say "oh es ten". It's hard to break the habit after 5+ years though.
If it was really meant to be "OS 10", then they wouldn't have versions like "OS X 10.6", they'd just have "OS X.6" or "OS X.VI". How many people do you hear saying "OS ten ten point six"?