I've been working with organic groups and the core forum module for about a week, and I'm generally pleased with how they interact. I'm getting stuck, though, trying to implement one of my key requirements.
Here's what I'm trying to accomplish:
1) All groups share the same forums. (easy)
2) Forums page shows all posts to anonymous users. (easy)
3) Forums page shows authenticated users only posts to their subscribed groups. (where I'm getting stuck)
Some Background:
1) Solving the first requirement was easy. All I needed to do was create forum areas using the core forums module, and all of my groups were able to post to these forums. This seems to be the normal behavior for the og and core forums modules.
2) The second requirement wasn't difficult either. First, I verified my settings under administer / access control to make sure anonymous users had privileges for "node module: access content" and "comment module: access comments". Next, I went to administer / settings / og and made sure "visibility of posts" was set to "Visible within the targeted groups and on other pages" (which has the effect of marking every post as "Public"). This combination of settings allows search engine spiders and other guest visitors to see plenty of posts when they visit the forums page.
3) Now the part where I'm getting stuck -- I keep running into an apparent conflict between requirements #2 and #3: