An Offline OST file is created by MS Exchange, if you want to open Offline Exchange OST file folder in MS Outlook, try OST Recovery software that restore offline OST file with all email properties and export them into PST file format which can be easily readable in Outlook mailbox. This software also Export Offline OST file to PST, Live Exchange Mailbox and Offline 365 mailbox.

https://www.linkedin.com/pulse/100-safe-ost-pst-converter-software-ostrecovery-kathy-rose
 

If you want to repair corrupted or inaccessible PST file, you can go with one of the best Outlook PST File Repair tool to easily fix corrupted MS Outlook PST file and successfully scan them to save in new workable PST file format with all email properties. This software also export corrupte PST file to Direct Live Exchange Mailbox, Archive Mailbox and Office 365 with all email properties.

http://download.cnet.com/Voimakas-Outlook-PST-Recovery/3000-2369_4-77402907.html