If you want to convert OST file to PST format, you can use few easy manual tricks that require no 3rd party tool. Microsoft Outlook client has few built-in features which you can use here, such as Archive, Export, Move, and Drag & Drop. By using these features, you can successfully import mailbox content from OST to PST file. Refer to this article for complete information - Convert OST to PST Format using 3 Easy Manual Tricks

But before you can use any of the manual tricks shared in the above article, make sure:

You're using the same computer system on which the OST files were originally created.

You're connected to the Microsoft Exchange Server.

And most importantly, the OST files are not corrupt.

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How to Convert OST to PST Outlook