i am using drupal 7.
I want to add forums in OG groups so that forums added in OG groups can only be accessible by members of the OG groups.
Is there any other option to grant access restriction on forms except using Form Access, because it will not integrate groups with forums?
Not currently. There was the OG Forum module for Drupal 6, but it is no longer maintained. There is an open issue about porting it to Drupal 7, but I'm not sure how likely that is at the moment: http://drupal.org/node/1007700
Your best bet might as well be to create your own custom content type and simulate forum behavior through comments or something like that....
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There is currently this effort for Drupal 7: http://drupal.org/project/og_forum_D7