I’ve just published the Organic Groups Privacy module to Drupal.Org. It’s an API module intended to help give developers an easy and flexible way to define public access to Organic Groups posts. It exists specifically so I can convince Spaces that I really mean it when I say an arbitrary 90% of an Organic Group should be private.
It is bundled with a Feature that demonstrates how you might integrate it with an Open Atrium site, but the core of OG Privacy can be useful even without Spaces being in the mix at all.