I love
Octavia, and yes, I think it's usable. I used to work with an
Octavia. But even before I'd read the body of your message, I was thinking about its lack of nickname potential and was going to mention that as a possible negative. Usually I'm one to say that for long names, nicknames, or perhaps not nicknames per se, but "shortenings" such as Oct, are inevitable. But the
Octavia that I worked with was always called
Octavia, as far as I ever heard. So I'm not sure about this one.
Using it as a middle name for a child born on the eighth day or in the eighth month is a cute idea. I think
Princess Beatrice of
York should have been given
Octavia for one her names, since she was born on 8/8/88.