How to fix .OST file Error

1. In Outlook, Go to File in the top left-hand corner.
2. Go to Account Settings.
3. Head to the Data Files, then Open File Location...
4. Once File Explorer opens, close all instances of Outlook. Delete the .ost file while Outlook is closed.
5. Once you have deleted the .ost file while Outlook is closed, open Outlook again and it will recreate the .OST file.