Here are the general steps you can follow:
Connect Outlook to Exchange Server: Ensure that you are connected to the Exchange Server, as this will be required to export the OST file.
Export the OST file to a PST file: In Outlook, go to File > Open & Export > Import/Export, select "Export to a file," and then choose "Outlook Data File (.pst)" as the export type. Select the OST file to export, choose a location to save the PST file, and follow the prompts to complete the export process.
Greetings,
Peter