OST file is automatically created when we connected MS Outlook client to MS Exchange Server. OST (.offline storage table) files allow users to work in offline mode, that is, when there is less or no network connectivity available. And when network connection gets connected, OST files get synchronized with the user mailbox on MS Exchange Server.

But sometimes you may face an error such as synchronization failure or unable to open ost file which gets corrupt or becomes inaccessible due to Exchange server crashed or ost file infected by Virus and many more reasons. In that situation, you need third party application to perform OST to PST conversion process that will help you in recovering your email data from corrupt ost file and also convert your ost file data to Outlook PST
file, where you can access all mail easily.

Here I shared a link: http://www.osttopstconverter-tool.com/