Offline Data File (OST)
Typically, when you use a Microsoft Exchange Server account (Office 365 account), your email messages, calendar, and other items are delivered to and saved on the server. You can configure Outlook to keep a local copy of your items on your computer in an Outlook data file that is named an offline Outlook Data File (.ost). This allows you to use Cached Exchange Mode or to work offline when a connection to the Office 365 (Exchange) computer may not be possible or wanted. The .ost file is synchronized with the Office 365 (Exchange) computer when a connection is available.