As seen on the Oni Central forum, Bungie's Matt Soell responds to the question of whether a post-release patch or third-party hack could add multiplayer to Oni:

I very much doubt there would be any way of hacking the finished app to enable multiplayer.

The idea of adding multiplayer to the game at some later date has been discussed, but right now our priority is finishing the game. There is no definite plan right now for the addition of any major features post-release. I couldn't rule it out, but I can't promise it either.

Dare we hope for a patch? Check out the whole thread.