What is OST File Extension?

OST File Default Locations: :

• Windows 8: Drive:\Users\user\AppData\Local\Microsoft\Outlook
• Windows 7 / Windows Vista: Drive:\Users\user\AppData\Local\Microsoft\Outlook
• Windows XP: Drive:\Documents and Settings\user\Local Settings\Application Data\Microsoft\Outlook

Detailed Information for what is OST File Extension!

The OST file format is an Outlook data file which stores email messages, attachments, contacts, tasks, calendars, journals, notes and other data hoarded by the application from an Exchange server (an e-mail-based mail and calendaring server developed by Microsoft).

OST files is also stands for offline storage table which is a copy of an Exchange mailbox generated at the local end when the user configured MS Outlook with cache exchange mode. Generally OST extensions help users to work offline even the Exchange server is dismounted or down. We hope now you will be aware about what is OST file extension. Read continuing to check free software to open ost files.

Note: If you don't check "Cache Exchange Mode" while configuring MAPI account, then the data will directly sync from Exchange server instead of OST file.

OST Format Internal Structure and Size Limitations

Above section evaluates what does OST stand for outlook and now check its structure. OST files stores the mail data in B-tree form and data is organized hierarchically in the form of leafs and nodes (512 bytes). It has a similar structure as .pst file extension.

1. Outlook 2002 and earlier editions use ANSI format encoding & 32-bit pointers. This file structure had the maximum of 2 GB file size limits.


2. MS Outlook 2003 and later editions use Unicode format for Unicode (UTF-16 little-endian) & 64-bit pointers. This file structure overcomes the 2 GB limit and specified standard size limits for each Outlook edition.

Note: To read binary values of emails, you can open .ost file extension with edit plus or Hex editor program.

Software that Opens OST file Format!

Program to Read OST files in Windows OS

1. Microsoft Exchange Server
2. Microsoft Outlook
3. Free OST File viewer

Utilities to open .ost extension in MAC OS X:

1. Microsoft Outlook for Mac

Related Article: How to Convert OST to PST Format