What is OST File?

Overview – What is OST File Extension?

An Offline Storage File (OST) or Offline Data File format used by Microsoft Outlook to store the Email application data. Offline storage file is created when Microsoft Outlook used with Microsoft Exchange Server.

It also holds all Email messages, notes, journals, tasks, contacts and calendar in a single file.A local file created in a computer in Outlook data file named as Offline storage file. This file used when we work in Cache exchange mode or want to work offline, without connection with Microsoft Exchange Serve

Local OST File is the slave replica of Exchange server Mailbox. File get synchronized when a connection is available.

Category of Application

  • The OST file stored the complete items into it therefore it is considered as Database file.

Application Software

The OST file is only supported by the Microsoft Outlook When connected with Exchange Server in cache exchange mode



File Signature:-

It has the following file signature:- hexadecimal 21 42 44 4e


To provide the facility to work with emails in ofline mode. Microsoft has introduced the concept of OST file format.

Location of OST files

Microsoft Outlook 2016
C:\Users\%username%\Documents\Outlook Files.

Microsoft Outlook 2013
C:\Users\%username%\Documents\Outlook Files.

Microsoft Outlook 2010
C:\Documents and Settings\%username%\My Documents\Outlook Files

Microsoft Outlook 2007

Microsoft Outlook 2003
C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook\

Microsoft Outlook 2000
C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook\