https://docs.microsoft.com/en-us/outlook/troubleshoot/data-files/cannot-change-the-location-of-ost-file


1. Remove Outlook profile in control panel - mail.

2. Flush all files inside of c:\users\username\appdata\local\microsoft\outlook and c:\users\username\appdata\roaming\microsoft\outlook.

3. Edit registry as per article and use the following path \\sis-fs03\ost\username  (create that subfolder on that network location).

4. Start and configure Outlook.

5. Close Outlook and configure for cached mode.