OST File (.ost)

An OST (Offline Storage File) is a computer file that stores data, such as emails and contact information, from Microsoft Outlook onto a user’s local computer. To download a copy of their Outlook data on their local computer, users must select the ʻUse Offline Folders’ option from their Outlook desktop app. This saves all Outlook data to a user’s computer hard drive using the file extension .ost. Business professionals that travel often use OST files when their internet connectivity is limited or unreliable, as OST files enable them to create calendar appointments, search or add contacts, and read and write emails using Microsoft Outlook while offline. However, any emails written or changes made to their OST files will not synchronize with the user’s online account until they have reconnected to the internet.