Microsoft Outlook's OST mailboxes can be exported or converted to PST files using the built-in export feature, and the following are mentioned further:
Launch Microsoft Outlook and go to the "File" menu.
Access "Open & Export" and choose "Import/Export."
In the Import and Export Wizard, select "Export to a file" and proceed.
Opt for "Outlook Data File (.pst)" as the file type and select the desired OST mailbox or folders for export.
Specify the location and name for the exported PST file, then complete the export process by clicking "Finish."