If the Outlook app is not closed properly, it can create problems for the .ost file. The following error is a common result:
If this error occurs, do not follow the instructions within the error message. Instead, close Outlook and let it completely close by itself. Do not force quit the application.
To check to see if Outlook has fully closed, look in the system tray.
Look for the Outlook symbol to be grayed out as shown below.
If the icon is there, do not try to restart the computer or force quit the application. Only after the icon disappears should one attempt to reopen Outlook. The system tray without the icon is shown below; the applications within the system tray will vary, but notice the absence of the Outlook application's icon.
Outlook will take longer to load upon reopening the application.