There are a few manual ways to convert OST to PST for free, but they have certain problems. Using Microsoft Outlook itself is one of the most prevalent free ways to do this.

Free Manual Method (Using Outlook)

You can export your OST file to PST if it is linked to an Outlook profile that is currently open:
On your computer, open Outlook.
Click on File, then Open & Export, and then Import/Export.
Choose "Export to a file" and then click "Next."
Select the Outlook Data File (.pst) option.
Choose the mailbox folders you want to export.
Select a location for the destination and click Finish.

This method is free, but it only works if the OST file is linked to Outlook and can be accessed. If you have orphaned or damaged OST files, you need a professional program like TrustVare OST to PST Converter.