When Outlook fails to import an OST file directly, the first step is to convert the OST file to PST format, as Outlook only supports PST for imports. You can use the Outlook Import/Export Wizard to manually export your data, but this requires access to the original Outlook account. If direct access is not possible or if the process seems cumbersome, consider using a specialized tool like Shoviv OST to PST Converter. This software simplifies the conversion process, allowing you to recover and import emails, contacts, and calendars from an OST file into Outlook efficiently. It is particularly useful for dealing with orphaned or inaccessible OST files, ensuring a smoother transition back to functionality.
Read more: https://www.shoviv.com/blog/how-to-import-ost-file-into-outlook-2016-2019-2013/