It was never really meant to be "OS 10", despite what the 'correct way' to say it was. Pure marketing.
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"?
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"?