I've had a think about this and I'm still of the opinion that OOG is beneficial to the game in many ways. So with that in mind I've decided to follow Mafia's example and leave it up to hosts of each game:

Allowed: OOG is allowed with no restrictions.

Restricted: OOG is allowed, but the host will specify restrictions.

Banned: OOG is not allowed.

For restricted, the kind of thing I expect would be "it's allowed, but only with your team mate" in games where you're in teams of 2. Although the ideal situation in that example would be that the host gives each team a thread, I realise some people enjoy the much faster approach of using IMs/voice chat/etc.

DEFAULT - Banned

By default OOG is banned, if a host does not clearly specify then you must work on assumption that OOG has been banned for that particular game.