If you want to restore a lost database of Offline OST files while keeping the files in their original form, try vMail OST to PST converter software.  That automatically works to assist you in recovering Offline OST file database items such as emails, contacts, chats, notes, journals, appointments, tasks, contacts, and so on in a short amount of time. The software can export items from OST files to PST, EML, MBOX, CSV, vCard, MSG, HTML, and OFFICE 365 Cloud.

Know more info- https://www.vsoftware.org/ost-to-pst-converter.html