OST File Definition & Meaning

An OST file, or offline storage table (.ost) file, is an Offline Outlook Data File that stores a synchronized copy of mailbox information on a local computer. This type of file is used by Microsoft Outlook email applications, including Office 365, Exchange, IMAP (Internet Message Access Protocol), and Outlook.com.

OST files allow users without internet connection to view email resources and perform certain tasks. When an internet connection is restored, these files automatically synchronize any new items or files on the server that were updated in the local OST file.

These files are automatically created upon the first time a Microsoft email user connects to a server. The ability to work offline is valuable for those who travel often or regularly work in environments with limited or unreliable connectivity. This need has become even more important with the recent boom in remote work.

What is stored in an OST File?

OST files store can grant offline access to a number of email resources, including:

  • All emails
  • Calendar data
  • Contacts
  • Tasks
  • Notes
  • Hidden items and other “overhead”
  • Public Folder Favorites
  • Shared folders of additional mailboxes linked to an Exchange account

The most commonly used functions of OST files are to read and compose emails, create appointments on calendars, and add or change contacts. However, they are not able to send or receive email messages without a connection. Emails will be sent and received once a connection to the Exchange server is restored.

OST vs. PST files

Microsoft Outlook categorizes Outlook Data Files into two types: OST files and PST files. PST, or personal storage table (.pst), files contain data that is stored in your personal folders, whereas OST files contain data that is downloaded off the Exchange Server.

While OST files were created to synchronize Outlook accounts with email servers, PST files were created to set up accounts on new machines, such as PCs or laptop computers, or to create backups in the event files become corrupted or a storage device crashes. This is because Outlook can’t directly import files and data from an OST file like it can from a PST file.

If you need to copy offline data to another computer, you have two options:

  1. You can configure the email client to connect to your server and create a new OST file.
  2. You can convert your OST content to the PST format and export it to Outlook.




Kyle Guercio
Kyle Guercio has worked in content creation for six years contributing blog posts, featured news articles, press releases, white papers and more for a wide variety of subjects in the technology space.

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