Hi @Max Fury ,
What's the type of your account(IMAP or Exchange)? And why did you want to import an .ost file?
My Outlook suddenly stopped working and it can't directly import from an OST file like it can from a PST file
Please first understand .ost file stores the mailbox copy instead of specific message items like .pst file, it's not supported to import the .ost file to Outlook directly, which is by design and we couldn't change it, for more information, please refer to this official document: Introduction to Outlook Data Files (.pst and .ost)
As a supplement, it's actually not necessary to import .ost file in most circumstances, because lots of data(such as your messages) in your .ost file is also stored in your server( if you are working with exchange account, all data(messages, tasks ,contacts, calendars) is stored in your server), if you could log in to your mailbox normally , it's recommended to re-configure it directly in Outlook to generate a new .ost file and get the data.
If you still have any questions, please feel free to post back.
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