I have to disagree on side details: OS X did have version numbers starting with 10. I used it between Panther and Yosemite and I can remember Tiger being called version 10.4; it was literally printed on the installation disc.

Yes, that means “Mac OS X 10.0” had two tens right after one another. Apple aren’t Microsoft silly when it comes to names but they can be silly.

Also, it wasn’t introduced as OS X. It was “Mac OS X” until 10.7, then “OS X” until 10.11, and “macOS” since then.