The OST File Format: An In-Depth Overview

OST (Outlook Offline Storage Table) is a file format utilized by Microsoft Outlook to create an offline copy of an email accountís contents, including emails, calendars, contacts, tasks, and other data items. In this article, we will delve into the intricacies of the OST file format, its functionality, advantages, limitations, and more.

What Is an OST File?

OST files are generated when you configure an Outlook account with Microsoft Exchange, Office 365, or The file acts as a mirror of the data stored on the server, allowing you to access your information even when offline. Changes made while offline are synchronized when the connection is reestablished.

How OST Works

When you set up an email account in Outlook, the application downloads data from the server and stores it in an OST file if the account is configured to use Cached Exchange Mode. You can read, delete, and send emails, which get updated on the server the next time you go online.

Advantages of OST Files

Limitations and Risks

Common Uses

How to Manage OST Files

You can manage OST files from within Outlook under Account Settings. Options include changing the location of the OST file, controlling the amount of data to be cached, and more.

Converting OST to Other Formats

OST files are often converted to PST (Personal Storage Table) for more flexibility and portability. Numerous software solutions, both free and paid, can assist with this conversion.

Security Implications

OST files can contain sensitive information, so itís crucial to implement security measures such as encryption and periodic scans for corruption.


The OST file format is an integral part of Microsoft Outlookís functionality, offering offline access and improved performance. However, its benefits come with limitations like profile dependency and susceptibility to corruption. Understanding the OST format can help users and administrators make the most out of its advantages while mitigating risks.