Outlook Error Message "Cannot Open .ost File"
This error message usually appears when Outlook is opening. .ost files enable Outlook to be used if the computer is offline. The files usually exist and this error suggests the file is probably corrupt.
The simplest thing is to try to force Outlook to rebuild it. Do this by navigating to C:\Users\USER\AppData\Local\Microsoft\Outlook and move or rename the .ost file. Open Outlook and it will rebuild the file when it can't find it.
For good measure, or if that doesn't work, go to C:\Program Files (x86) and search for scanpst.exe. Run it to repair the files (try it on the .nst file as well).
There has been some experience of these measures not taking effect until after a reboot of the PC.