1. Open Calendar and press cmd-n. Type a name and press enter.
2. From the 'repeat' popup button, choose 'Custom'. Make a selection (I used the first Tuesday of each month).
3. Press enter, or vo-arrow or tab to the OK button and press space.
4. Press tab, and VO reports moving through the new event dialog as expected. The problem, though, is that pressing vo-arrows reports focus on the regular Calendar interface--toolbar, month area, etc--and not the dialog that the tab key is moving through. You can press vo-f2 twice, choose the dialog, and be back to normal, but you must do that to get focus working properly again. Basically, the popover is tabbable but VO stops focusing on it.
Bug First Encountered
How often the bug occurs
Vo-f2 twice, choose the event dialog, and things should be back to normal.