OST or Offline Storage Files represent the user's mailbox data in offline mode on the local machine upon registration with Exchange Server using Microsoft Outlook. It is automatically created on the first use of Microsoft Outlook upon connectivity with the server. Once the file is created, the data is synchronized with the email server so that it is available offline as well in case of disconnectivity from the email server.
A Tab-Separated Values (TSV) file format represents data separated with tabs in plain text format. The file format, similar to CSV, is used for organization of data in a structured manner in order to import and export between different applications. The format is primarily used for data import/export and exchange in Spreadsheet applications and databases.
You can also convert OST into many other file formats. Please see the complete list below.