In reply to Vanessa Anne Blaylock.

o2 has been available in Beta since December 2013, but only on WordPress.com. Beta testers were able to use o2 for public or private WordPress.com sites, and reported quite a few bugs allowing the o2 team to keep improving the product. We at Automattic also use O2s every day, for all internal asynchronous communications, as Sarah mentions in the post. That makes for quite a few additional Beta testers :)

My guess is that somewhere along the way Automattic decided they liked Slack better than O2

Slack and o2 are quite different. We do use both at Automattic, but one doesn't replace the other. Slack is built for synchronous chats, and has basically replaced IRC / Skype / HipChat. o2, on the other hand, is designed for asynchronous conversations. It's basically a mix between a forum and a blog.