From what I have read, because o2 will be a plugin, not a theme, it should be compatible with any theme in the same way that BuddyPress or bbPress can be activated and work on most themes. If you check out the http://geto2.com/field-guide/ link shared above, it indicates that "Breathe is an o2-compatible theme based on _s" which one would assume means there will be other "o2-compatible" themes down the line.

I've been holding my breath waiting for o2 for a long time now. I hope it gets released to the wild shortly.