To manually convert an OST file to a PST file, first, open Microsoft Outlook and go to "File" > "Open & Export" > "Import/Export." Select "Export to a file," choose "Outlook Data File (.pst)," and then select your OST file to export.

Alternatively, you can use Sysinfo OST to PST Converter which is the finest and most efficient utility for converting an OST file into an Outlook PST file with 100% accuracy and no data loss is a versatile software that not only converts OST to PST but also supports other formats such as MBOX, PDF, JSON, and more. This multi-functional tool ensures accurate conversion and preservation of data.
https://www.sysinfotools.com/ost-to-pst-converter.php