OST files are very important files as they contain the whole mailbox data. They act as backup files of user mailbox items. They help user to work also in offline mode when there is no connection between MS Outlook and MS Exchange server. When the connection between them re-established, they get synchronized with earlier files.
OST to PST Converter
OST or Offline Storage Table is the local storage area of MS Exchange Server that locally stores data on the storage device. This offline folder is an integral component of Exchange Server and needs to be maintained in order to keep smooth communication while accessing Exchange Server. Since it is prone to corruption, any single case like server crashing or mailbox deletion from the Exchange Server could create problem making it inaccessible or unusable. When such thing happens, a large part of user's data becomes inaccessible.