Best method is to do the following OST to PST process:
1. Open MS Outlook
2. Click the File Menu
3. Select the “Import and Export” wizard or press ‘t’.
4. From the list, click the “Export to a file” option and then click Next button.
5. Select Personal File Folder (.pst) and then click Next button.
6. Select the items to be exported or Click on parent root and check the Include sub-folders option.
7. Browse and select the target folder for saving the email items.
8. Click the Finish button.