If you're looking for free methods to convert OST files to PST format, it's important to understand that Microsoft Outlook does not provide a direct built-in option for this conversion. However, if your OST file is still accessible and linked to an active Outlook profile, there are a few manual workarounds available:

Manual Methods to Convert OST to PST:

Export via Outlook Import/Export Feature
Use Outlook’s built-in Import/Export function to export your mailbox data to a PST file.

Use the Auto-Archive Feature
Navigate to Advanced > AutoArchive Settings and set archiving rules to move mailbox items into a PST file.

Drag and Drop Method
Manually create a new PST file in Outlook and drag mailbox folders or emails from the OST file into it.

Recreate OST and Backup Data
If needed, you can delete and recreate the OST file. Before doing this, use the Export or Archive feature to back up data to PST format.

Limitations of Manual Methods:

While these options may seem cost-effective, they come with significant drawbacks, including:

❌ Time-consuming and labor-intensive, especially for large or multiple mailboxes.

❌ Requires technical knowledge to avoid errors during export.

❌ High risk of data corruption or loss, particularly if the OST file is damaged, orphaned, or disconnected from Outlook.

❌ Only works when the OST file is accessible via Outlook. You cannot use these methods for orphaned or corrupted OST files.

The Safer Alternative: Use a Professional OST to PST Converter

When dealing with orphaned, damaged, or inaccessible OST files, or when you need to perform a large-scale or risk-free conversion, using a professional OST to PST converter tool is the recommended approach. Though it's a paid solution, it ensures:

✔️ Complete data integrity

✔️ Support for all types of OST files

✔️ Multiple export formats

✔️ Easy-to-use interface for all users

In conclusion, while free manual methods exist, they’re best suited for basic needs and accessible OST files. For anything beyond that—especially when data security and accuracy are a priority—a dedicated OST to PST converter tool is the smarter and safer choice.
https://www.goldytools.com/blog/ost-file-corruption-reason-and-solution/