I recently have been working with OA in a multisite setup and am having an interesting issue with filepaths for uploaded items. Admittedly, my debugging skills leave much to be desired but I wanted to address it here. I have never had this problem with single site installs.
In admin/settings/file-system my file system path is set to a standard location sites/example.com/files. When I upload a file with a event, blog, etc. the file is uploaded correctly onto my server in the expected location. Additionally in the 'Attach files to this event' dialog box, the path below the file name is correctly displayed. However, the link generated with the file preview itself includes extra path information such as http://example.com/groupname/%252sites/example.com/files/image.jpg. Obviously, this leads to a file not found error. This same erroneous link is rendered on the actual event, blog, etc. node.