Although Mullenweg says anyone on WordPress.com can enable o2, this is not the case. Searching for o2 on WordPress.com generates no results while P2 brings up the original P2 theme.

Anyone who'd like access to the "new" P2 on WordPress.com (aka O2) is welcome to make a request in the Themes forum and we'll be glad to get you set up. We'll also add you to the beta users group.

https://en.forums.wordpress.com/forum/themes

Also to clarify, O2 on WordPress.com is called P2 now, while the old P2 was renamed P2 Classic.