organic group forum restricts viewing forum nodes by group membership and block any use of a general forum setup

 FYI -> have made an issue on Drupal.org for this: http://drupal.org/node/981582

 Just shortly, using OG Forum which is not in the Commons distribution for now, 

 AFAIK you will be no more able to build a general Forum Container and a Forum that can be accessed by authorized users.

 The main bug/or by design  issue is that you cannot choose a forum trying to add a topic  in the form not being joined to any Group.

 So only if you joined a group you will have the possibility to add topics to the Group-Forum.