Converting OST files to PST files can be accomplished using various methods. Here's a general guide on how to convert OST to PST files:

Use Outlook's Import/Export feature:
    Open Outlook.
    Go to "File" > "Open & Export" > "Import/Export".
    Choose "Export to a file" > "Outlook Data File (.pst)".
    Select the folders to export and save the PST file.

Use a third-party OST to PST converter: Kernel for OST to PST Converter https://www.nucleustechnologies.com/ost-to-pst-converter.html
    Download and install a reliable converter tool.
    Open the tool, select the OST file, and choose the destination for the PST file.
    Start the conversion process and wait for it to complete.

https://www.nucleustechnologies.com/ost-to-pst-converter.html