https://support.microsoft.com/en-us/kb/2752583

To set the ForceOSTPath registry entry in order to change the location of the .ost file,follow these steps:

1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Locate and then select the following registry subkey:
   HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook

   Note The xx.0 placeholder represents your version of Office (16.0 = Office 2016, 15.0 = Office 2013).

3. Right-click Outlook, select New, and then click Expandable String Value.
4. Type ForceOSTPath, and then press Enter.
5. Right-click ForceOSTPath, and then click Modify.
6. 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.
7. On the File menu, click Exit to exit Registry Editor.