(Office 365) (Windows) (Outlook 2013) (Outlook 2016) Solve an Outlook error by deleting the .ost - the Outlook data file or cache.

If you see a message such as ""Outlook is using an old copy of your Outlook data file..." when you open Outlook, the file in question is the last cached copy of an Office 365 mailbox, also known as the .ost file . The message means that Outlook can't open that file for some reason, perhaps an Outlook or Windows crash. Because the file in question is a cache, it's safe to delete it because the real copy of your mailbox is on the Office 365 server. Below are instructions to find and delete the .ost file.

The Outlook cache is a hidden file in a hidden folder, so first thing to do is to unhide it.  

File > Exit from Outlook, if it is still open.  

For Windows 7

      • Open Computer from the desktop or Start button.
      • Click File > Organize > Folder and Search Options
      • Select the View tab
      • Tick the option to "Show hidden files, folders and drives"
      • Uncheck the option to "Hide extensions for known filetypes"
      • Click OK.

For Windows 10

      • Open File Explorer
      • Click View
      • Check the boxes for "File name extensions" and "Hidden items"
Now, in the Computer or File Explorer window, navigate to
C:\Users\<your login name>\AppData\Local\Microsoft\Outlook
You may delete any files with the .ost extension.  (Do not delete any files with the .pst extension)

Now you should be able to open Outlook.  Be mindful that you've just deleted all your cached email, so Outlook will now take some time recreating that cache.   You will see the "updating folder" notification in the lower right of the window until all cached folders are up to date.

See also:  https://kb.wisc.edu/page.php?id=28858 for a detailed explanation of these Outlook files and other important information.

Keywords:.ost Office 365 error message cache delete   Doc ID:60675
Owner:Adrian G.Group:School of Medicine and Public Health
Created:2016-02-11 09:43 CDTUpdated:2016-02-24 09:35 CDT
Sites:School of Medicine and Public Health
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