https://support.microsoft.com/en-us/help/2752583/
To set the ForceOSTPath registry entry in order to change the location of the .ost file,follow these steps:
Click Start, click Run, type regedit in the Open box, and then click OK.
Locate and then select the following registry subkey:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook
(16.0 = Office 2016, Office 365 and Office 2019, 15.0 = Office 2013).
Right-click Outlook, select New, and then click Expandable String Value.
ForceOSTPath
Right-click ForceOSTPath, and then click Modify.
In the Value data box, type the full path of where you want to store the .ost file (such as D:\MyOST), and then click OK.
On the File menu, click Exit to exit Registry Editor.