I had OG working fine except I wasn't able to upload images to be displayed (but to still keep the File field for PDF uploads as private to just the group).
However I've gone through and removed the CKEditor module as people suggested to use WYSIWYG with CKEditor (js library) so I moved that.
Then I installed the IMCE - WYSIWYG bridge too and tried several things to get the images working.
They were working but I noticed PDFs were no longer private, now I seem to have narrowed it down to the OG field settings.
If I add the "Group Audience" field to the content type (which is set as OG group content), then I can no longer visit that page and get this error:
EntityMetadataWrapperException: Unknown data property og_group_ref. in EntityStructureWrapper->getPropertyInfo() (line 339 of sites/all/modules/entity/includes/entity.wrapper.inc).
As soon as I remove the Group Audience field, I can access the content type again (i.e. I can edit, add new etc. for this one content type). However I need the Group Audience field to keep the content secure/private.