After enabling the og_field_access module, none of the additionally introduced permissions are granted to anyone. This can be changed in the "Organic Groups field access" section of "Administration -> Configuration -> Organic groups -> OG permissions Overview" (/admin/config/group/permissions).

However, because no permission is granted, access to site content will not work as before. For example, because "View Body field" is not granted, only site admins will be able to see the content of posts, wikis and other group content, while anonymous users etc. see only the title of these.

I propose to initialize these additional permissions to correspond to the current state of permissions for the whole (public or private) group. Because when enabling a module, a Drupal admin (I think) is quite used to get additional functionality from the module and not loss of it. (That's why it took me a day to track down why the body field does not show any more; admittedly including own stupidity because I had forgotten about enabling og_field_access.)