File with an .OST extension contains locally stored data downloaded from the Exchange server with Microsoft Outlook. It may be email messages, contacts or attachments also of large volume, unlike the similar PST files (Personal Folder), which by default support smaller volume files. The maximum volume of both files can be changed with proper entries in the register.

OST files are used by MS Outlook programs from the 2005 version, if the Offline catalogues option is chosen, which enable access to the data without necessity to connect to the Internet. In such case, they are stored on a local drive.