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Last updated: April 8, 2026

Quick Answer: Deleting an OST (Offline Storage Table) file is generally safe if you are no longer using that specific Outlook profile or if you have a reliable backup of your Outlook data. However, it's crucial to understand that an OST file is a synchronized copy of your mailbox data from an Exchange, Office 365, or IMAP server, and deleting it will remove this local copy. Ensure you have a PST file backup or that your mailbox is still accessible online before proceeding.

Key Facts

An OST file is a synchronized offline copy of your Exchange, Office 365, or IMAP mailbox data.
Deleting an OST file removes the local copy of your emails, calendar, contacts, and tasks.
You can safely delete an OST file if you have a PST backup or if your mailbox data is still available on the server.
If your Outlook profile is no longer active or you are migrating to a new profile, deleting the old OST is often necessary and safe.
Corrupted OST files can sometimes be resolved without deletion by using tools like ScanPST.exe.

Overview

The question of whether it is safe to delete an OST file often arises for users of Microsoft Outlook, especially those who manage multiple email accounts or have experienced issues with their Outlook data. An OST file, or Offline Storage Table, serves as a synchronized cache of your mailbox data when using Exchange, Office 365, or IMAP accounts. This allows you to access your emails, calendar entries, contacts, and tasks even when you are not connected to the internet. Outlook creates this file automatically when you set up an account that uses these protocols, ensuring a seamless user experience.

However, the presence of these files can sometimes lead to confusion, particularly when dealing with storage space or troubleshooting Outlook problems. Understanding the role of an OST file is paramount to determining the safety and consequences of its deletion. It is not a primary archive like a PST (Personal Storage Table) file, but rather a dynamic, server-linked repository of your mailbox content. Therefore, the decision to delete an OST file requires careful consideration of your current Outlook setup and data management practices.

How It Works

Synchronization: The primary function of an OST file is to facilitate offline access and synchronization. When you make changes to your emails, calendar, or contacts while offline (e.g., send an email, mark an item as read, or create a new appointment), these changes are stored locally in the OST file. When your internet connection is restored, Outlook synchronizes these changes back to the server, ensuring consistency across all your devices and access points.
Offline Access: Because the OST file contains a copy of your mailbox, you can continue to work with your emails, schedule meetings, and manage contacts even without an active internet connection. This is a significant advantage for users who travel frequently or work in environments with unreliable network access.
Server Dependence: It's vital to remember that an OST file is fundamentally dependent on its associated email account and the server it synchronizes with. If the account is deleted from the server, or if the server connection is permanently lost, the OST file becomes inaccessible and effectively orphaned.
Not a Backup: Unlike a PST file, which is typically used for archiving and can be manually created and backed up, an OST file is a live, synchronized copy. It is not designed as a long-term backup solution. If the OST file becomes corrupted, or if the associated account is removed, the data within it is generally lost unless there's a server-side backup or a PST export.

Key Comparisons

Feature	OST File	PST File
Purpose	Offline cache for Exchange, Office 365, IMAP accounts; synchronization with server.	Local archive for POP accounts, historical data, or manual backups.
Creation	Automatically created by Outlook upon setup of compatible accounts.	Manually created by user for archiving or export.
Data Source	Synchronized copy of server-side mailbox data.	User-added or exported data; independent of server unless linked to POP account.
Offline Access	Enables full offline access to mailbox data.	Provides access to data stored within the file, regardless of server connection.
Deletion Consequence	Removes the local copy of synchronized data; data can be recovered from server.	Removes the archived data permanently if no other copy exists.
Server Dependency	Highly dependent on the active email account and server.	Independent of email server unless configured for POP accounts.

Why It Matters

The ability to safely delete an OST file can have a significant impact on your computer's performance and your Outlook experience. Large OST files can consume substantial disk space, potentially slowing down your computer. In 2023, it's not uncommon for OST files to range from several gigabytes to over 50 gigabytes for users with extensive email histories.

Furthermore, corrupted OST files are a frequent cause of Outlook crashes and data access issues. While Outlook provides tools like ScanPST.exe to repair these files, sometimes the corruption is too severe, or the user may wish to start fresh with a new profile. In such scenarios, knowing when and how to safely delete an OST file becomes crucial for troubleshooting and maintaining a healthy Outlook environment.

Finally, for users who are migrating to a new computer, changing their email provider, or consolidating their Outlook profiles, deleting old OST files can be a necessary step in the cleanup process. It ensures that you are not carrying unnecessary data or conflicting configurations forward, leading to a cleaner and more efficient Outlook setup.

In conclusion, deleting an OST file is generally safe under specific conditions. The key is to confirm that your mailbox data is either still accessible on the server or backed up elsewhere. If you're unsure, it's always best to create a PST export of your Outlook data before proceeding with any deletion. This simple step can prevent the accidental loss of valuable information and provide peace of mind.