Know What is Microsoft Outlook OST File Format

MS Outlook maintains data in two ways one is in .PST format and second is in .OST format. OST is similar to PST but OST can be used only in the presence of MS Exchange sex toys
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Specification: –

.OST Extensions File Specifications

Developer: Microsoft
Format Type: Text and Binary
File Category: Email and Database
MIME Type: Unspecified
File Hex Signature: 21 42 44 4E
ASCII code. !BDN Program
Supported Platforms: Microsoft Windows and Mac

What is .ost File Extension:-

In MS Outlook OST stands for Offline Storage Table File which stores the complete data of Outlook like emails, calendar events, contacts, and tasks etc. The OST file is created when a user is using the Account of Exchange Server in MS Outlook with having Cache Exchange Mode is Enabled. If the Cache Exchange Mode is Disable then Outlook would not store the data in OST file, it will directly sync. the data from Exchange Server.

Also When a user is configured with IMAP Mail Server in MS Outlook 2013 and MS Outlook 2016 an OST file has been created and when a user uses Hotmail an OST file also created; for Hotmail, a user can use any MS Outlook version.

.OST file Extension Size Limitation: –

  1. MS Outlook 1997, 1998, 2000, 2002 use ANSI Encoding format and 32-bit pointers. The maximum file size limit was 2 GB for OST file extensions.
  2. MS Outlook 2003, 2007, 2010, 2013, 2016 use Unicode Encoding format for Unicode and 64-bit pointers. The maximum file size limit for the .ost file in Outlook 2007 is 20 GB. In Outlook 2010/2013 and 2016, the size limit has been extended to 50 GB.

What are the OST File Default Locations:-

  • MS Outlook 2003:- C:\Document and Setting\Username\Local Setting\Application Data\Microsoft\Outlook\Filename.ost.
  • For MS Outlook  2007:C: \Users \User Name \App Data \Loca l\Microsoft \Outlook \Filename.ost.
  • For Outlook2013/2016:C:\Administrator\App Data\Local\Microsoft\Outlook\ Filename.ost

Software required to open OST File:-

  • Microsoft Exchange Server.
  • Microsoft Outlook.
  • Free OST File Viewer.


In this Article, we have discussed the OST file extension. If you have any related doubt please feel free to comment or you can also create a separate discussion in our Official Outlook Forums. We will be happy to help you.

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