OOT stands for "Out Of Thread," and is almost exclusively used in the context of communicating with partners outside of the thread, such as your fellow mafioso or mason buddies. If you are allowed to communicate OOT with other players, typically you may do so freely: any time of the day at any point in the game, so long as you are both alive. Likewise you may take advantage of any medium you like: PMs and email are the most common, but it can and has included googledocs, IRC, AIM, even telephones or real life conversations.
If you cannot communicate OOT, then it is against the rules to do so. So be sure your PM says you can communicate OOT before you do so.