What Is OST? Understanding Offline Storage Table Files

OST, or Offline Storage Table, is a file format primarily used by Microsoft Outlook to store a local copy of mailbox data. The OST file allows users to work in 'offline mode,' offering the capability to read, delete, and even compose emails without an internet connection.

Why OST Exists

The primary reason for the existence of OST files is to ensure seamless data access. It was developed to provide a solution for situations where network connectivity is poor or sporadic. With an OST file, you can work on your emails, calendar, and tasks even if you are not connected to your Exchange server or Office 365 account.


People often confuse OST with PST (Personal Storage Table). Although both serve the purpose of data storage, they differ in several aspects:

When OST Files Get Corrupted

OST files are generally reliable but can sometimes become corrupted due to various reasons like abrupt termination of the Outlook application, hard disk errors, or even malware attacks. When this happens, it becomes necessary to either repair the OST file or recreate it by re-syncing with the server.

How to Access OST Files

OST files are typically found in the user profile directory on Windows systems. Their default location is under the 'AppData' folder. They are not meant to be manually moved or manipulated because they are tightly integrated with the user's mailbox on the Exchange server.

Best Practices for OST File Management

  1. Regular Backup: Although OST files sync with the server, it's a good practice to have a backup of important data.
  2. Monitor Size: Large OST files can slow down Outlook performance. Monitor the size and archive old items to keep it manageable.
  3. Corruption Repair: Utilize in-built or third-party tools to repair a corrupted OST file.


Understanding the purpose and functionality of OST files can enhance your experience with Microsoft Outlook, especially if you often find yourself working in environments with limited internet connectivity.