You can use a third-party tool to convert the data from an OST file to PST format.
If you're looking for a Free OST to PST Converter Tool, I recommend the OST to PST Converter appliance.

1. Install the OST to PST Converter and then click the Add Button to add the OST file.
2. Select the export option from the drop-down list after uploading the file.
3. Before you do that, make sure you can scan and inspect your OST files.
4. Now go to the location where you wish the exported file to be saved.
5. After that, you can convert your OST file to PST for free.