(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.
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"
C:\Users\<your login name>\AppData\Local\Microsoft\Outlook