.OST files can only be accessed via Outlook when a user is authenticated to use the .OST that has the appropriate mailbox GUID. Basically, you need to configure the original mailbox into Outlook and you can tell it to reference that .OST file. Although if the mailbox data is still in Exchange, Outlook will just re-cache the data and you don't really need to reference an old .OST file.
If this is not possible, you need to look at converting it to a .PST file, using a third party website. Just google OST to PST conversion, there are a few out there.
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