OST to PST converstion.

Conversion of the OST file to PST file would depend on the current status of the Outlook/Exchange account.

In case your OST file is linked to the Outlook profile, you could just go ahead and open the Outlook application and use its Import/Export feature to extract the mailbox data into PST. However, in case where the OST file is orphaned, corrupted, not accessible or the initial Exchange/Microsoft 365 account is no longer accessible, the Outlook software cannot be used for this task. In such cases, you will need a conversion tool to do this.

You might want to consider Shefware OST to PST Converter as one of the options available. The tool can scan and convert OST file to PST file, while dealing with orphaned or corrupted OST files as well. It can maintain the folder hierarchy and convert emails, attachments, contacts, calendar, tasks, and notes.