Some have claimed success with a bimac in a 30, but people around here strongly recommend something bigger. Bimacs can get pretty big and need space to play and hunt--some people on the forums here consider it animal abuse to keep such an intelligent animal in that small of a 'kennel.' 55 gallons seem to be the practical minimum.
The consensus seems to be that octo keeping is quite a financial and personal commitment, and if you don't have the resources to do it right, you shouldn't do it at all. If you're limited to 30 gallons, there are certainly some options for you.
Smaller octo species have been kept in 30 gallon tanks, however it might be a little bit more of a challenge. Bimacs are preferred because they're captive bred (you know EXACTLY what you're getting and how much natural lifespan is left, you're not finding out from some teenager at the pet shop) and they're diurnal. If you want to get a dwarf species for a 30, you're going to have to get one wild-caught that might only have a few months left ahead of it (or it could be a baby of something that might grow BIGGER than a bimac!). And there are plenty of wildcaught octos available on the internet.
If you're set on a 30, you don't have to see this as discouragement, but a challenge. People seem to be using red lights to see nocturnal octos, and to be honest, most bimacs do die within a few months, anyway.