What is .ost File Extension?

OST (offline data file) is a file format of MS Outlook that saves email messages, contacts, tasks, attachments, calendars, notes, journals, and other data stored by the email client from Exchange server.

OST file stands for Offline storage table that is mainly the copy of MS Exchange mailbox made at the local end at the time when users configured Outlook through cache Exchange mode. Usually, .ost file extension makes easy for users to work offline even when Exchange server is down or dismounted.

Specifications OST File Format

Developer Microsoft
Format Type Binary & Text
File Category Database & Email
ASCII Code !BDN Program
Supported Platform MS Windows & Mac
MIME Type Unspecified

Default Locations

The Outlook OST data file are stored at different location

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

Internal Structure and Size Limitations of OST

Above section, values that what exactly OST stand for Microsoft Outlook and now understand its complete structure. OST file format stores its email data in the form of B-tree and the whole data is set hierarchically in the method of nodes and leafs (512 bytes). It has related structure just like PST file.

Outlook 2002 and later version supports the ANSI format that is encoding along with 32-bit pointers. This structure of file had maximum limitation of 2 GB. Microsoft Outlook 2003 and earlier versions utilize Unicode file format for Unicode (UTF-16) with 64-bit pointers. This structure of file overcomes 2 GB limitation and quantified standard size limits for every Outlook version.

Note: To read the binary values of mails one can open and view Offline .ost file extension with an edit plus or Hex-editor program.


Data management is relatively significant in the same manner; there is an Outlook OST file that manages the complete offline data. In the above-discussion, deep about .ost files are deliberated, which makes easy for users to know it completely.