.OST File Extension
File TypeOutlook Offline Folder File
This file is saved in a binary format, which requires a specific program to read its contents.
Stores e-mail messages and other data saved by Microsoft Outlook (2005 or later) from an Exchange server; similar to a Personal Folder (.PST) File, but supports larger file sizes.
OST files are created when the "Use Offline Folders" option is selected in Outlook and are stored on the local hard drive instead of mail server.