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Export mailbox as a PST file
Login to Outlook with the Exchange account that was used to create the OST files. And to export the OST mailboxes to PST, follow the simple procedure as given here:

Go to Files >Open & Export and click Import & Export.Import & Export
Select Export to a file; then click Next.Select Export to a file
Select the second option—Outlook Data File (.pst)—and click Next.Select PST file
Select the folder(s) which you want to export and click Next.Select the folder to export
Note: You have the option to include subfolders as well.

Browse the location where you want to save the new PST files and choose an option regarding the duplicate items.
Note: There is an optional choice to provide a password to the PST file, but you can easily move ahead without choosing it.

The secure and easy way to transfer your Offline OST emails to PST is OST to PST Converter Tool. This application lets you quickly move emails from OST files to PST directly with attachments. Also, it maintains the folder hierarchy of OST mailbox folders while transferring into a PST account. It works well for moving both big and small email files safely. You can even transfer your emails straight to other email services like MBOX or EML, keeping all your files and details just like before. It comes with advanced options like filters to find specific email data and get a preview of your emails on any version of Windows. Refer: https://www.sametools.com/convert/ost/