Table of Content
Application Category & Support
Owned by Organization
MIME & File Signature
Relevant Action
Join Discussion

What is OST File Extension?

OST (.ost) stands for Offline Storage Table, also known as offline storage files in MS Outlook. OST files are the cache copy of an Exchange mailbox generated on the local end to work when the server is unavailable. Therefore, making OST file allows users to work in server downtime conditions which can later be updated to the Exchange Server the next time connection to the network is established. Moreover, OST files are replica of users mailbox stored in Exchange Server Database (EDB file). Everytime the connection to the server is available, the two files get synchronized with each other.

The OST file belongs to the category of Database Files.

All MS Outlook versions

Microsoft Corporation


Hexadecimal: 21 42 44 4e


The default address of OST file extension depends on the MS Outlook versions installed on the user system. For each version, the address will be different:

MS Outlook 2000 and 2003

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

MS Outlook 2007


MS Outlook 2010

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

MS Outlook 2013

C:\Users\%username%\Documents\Outlook Files

Hardware Reasons

  • Data Storage Device Failure
  • If your hard drive has some bad sectors
  • Networking Connection Failure
  • If your network connection has problems, then the synchronization process is aborted and OST file is possible to get corrupted.
  • Shutdown
  • If a shutdown occurs when you are opening or synchronizing OST format, then your it may get damaged.

Software Reasons

  • Terminate Outlook Unusually
  • If MS Outlook is busy and you terminate it or turn off the computer without exiting Outlook properly.
  • Synchronization Error
  • The failure of synchronization among the Offline Storage Table and the server might also lead to several errors.
  • Virus or Malware
  • Virus or Malware intrusion is capable of harming OST file format or make them inaccessible.

Miscellaneous Error