OST file, also called offline storage table file, is an offline Outlook Data File used by Exchange Server that enables you to work with mail or another data even when server access is not available for Server. OST files are created when Microsoft Exchange Server’s Cached Exchange Mode is enable. Default Location of OST File: Windows 10 & Windows 8,7: C:-Users-user-AppData-Local-Microsoft-Outlook- Windows XP: C:-Documents and Settings-user-Local Settings-Application Data-Microsoft-Outlook By default OST file is created by the name Outlook.ost