Hi Ryan,
Apologies for the delay in answering. Indeed, ^O is not implemented in ViEmu. You are actually the second person to request this, the other person was someone just getting introduced to vi/vim editing - I mention this because it's significative, as many hundreds of customers have not missed it. I understand that ViEmu should provide it at some point for completeness sake, but I believe that the want of using ^O is often a sign of *not* using vi/vim editing correctly. If I may say so, I'd like to recommend you to read my article about vi/vim editing, as I deal with the most efficient way to use insert and normal modes:
http://www.viemu.com/a-why-vi-vim.html
The main point covered there is that insert mode should be used just to insert short snippets of text, and one should go back to normal mode as quickly as possible. This is the most efficient way to use vi/vim editing, and if you do it that way, you don't miss ^O at all. I still haven't found a case where it's better to stay in insert mode and just issue an external command.
In any case, I take note of the request and I hope ViEmu will provide it at some point in the future, in which case I'll drop you a note about it.
And I hope you find my suggestion useful rather than disrespectful, as it's meant that way!
Best regards,
Jon