In order to import the OST files you can use the option Import/Export which is freely available in the Outlook. In Outlook, click on File from the toolbar, then click on Open & Export, now click on Import/ Export, this shall open a new dialogue box, select the “Export to a file” option then choose the option that reads “Outlook Data File (. pst)”. Then choose the folder to be exported, choose the place to save it, then click finish.

If this does not work, you can try using OST to PST converter by other third-party tools such as Shoviv OST to PST converter. Using this software, it is easy to convert files located in OST format into a format that can easily be read by Outlook, which is the PST format. It retains the context and form of the data storage i.e., emails, contacts, calendar, and others making it even easier. Due to its simple user interface and immense functionality, it is a perfect tool for import OST file to Outlook even if the mailbox of the original Exchange account is inaccessible. I recommended to attempt working with the FREE trial version to assess its potential and identify optimal strategy and methods.