What Is .OST

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Last updated: April 10, 2026

Quick Answer: .OST (Offline Storage Table) is a Microsoft Outlook data file that stores copies of emails, calendar items, and contacts from an Exchange Server account, enabling offline access. Introduced with Outlook 97 in 1997, OST files automatically synchronize changes back to the Exchange Server when the connection is restored. Unlike PST files, OST files are directly tied to Exchange accounts and can grow to several gigabytes depending on mailbox size.

Key Facts

OST (Offline Storage Table) was first introduced in Microsoft Outlook 97, released in 1997
OST files can range from 100MB to several gigabytes in size depending on the Exchange mailbox size and data stored
OST files support offline access to all Exchange mailbox items including emails, calendar events, contacts, and tasks without requiring a server connection
Unlike PST files which are static local archives, OST files maintain a two-way synchronization with Exchange servers, automatically syncing changes when reconnected
OST files are primarily supported on Outlook for Windows, with limited OST file support available in Outlook for Mac

Overview

.OST stands for Offline Storage Table, a file format created by Microsoft Outlook to store cached copies of emails, calendar items, contacts, and other data from an Exchange Server account. When you configure an Exchange mailbox in Outlook, the application automatically creates an OST file on your computer's hard drive, allowing you to access your emails and calendar even when disconnected from the network.

The OST file format was first introduced in Outlook 97, released in 1997, as a solution to the growing need for mobile professionals to work offline. Unlike PST files, which are manual local archives, OST files maintain an active connection to the Exchange Server and automatically synchronize any changes made offline back to the server when your connection is restored. This two-way synchronization ensures that your offline work is never lost and your mailbox remains consistent across all devices.

How It Works

OST files operate through an intelligent caching mechanism that downloads selected items from your Exchange mailbox to your local machine. When you first set up an Outlook client with an Exchange account, the system creates an OST file and begins downloading messages, calendar events, and contacts based on your synchronization settings. The file grows over time as more data is cached locally.

Offline Access: When you are offline, Outlook reads directly from the OST file instead of the Exchange server. You can read emails, view your calendar, and access contacts as if you were online, with all the same functionality available locally.
Synchronization Process: When your network connection is restored, Outlook enters synchronization mode, comparing the cached OST data with the current state on the Exchange server. Any changes made offline (deleted emails, new appointments, modified contacts) are sent back to the server, and any server-side changes not in the OST file are downloaded.
Cache Management: Exchange administrators can configure how much data is cached in OST files through retention policies and sync settings. Some organizations use "cached mode" to download the last 3 months of emails, while others download all available data depending on storage capacity and security policies.
File Storage Location: On Windows, OST files are typically stored in the user's AppData folder at C:\Users\[username]\AppData\Local\Microsoft\Outlook\. The file size directly depends on the amount of data configured for caching and can range from 100MB for minimal configurations to several gigabytes for power users with years of email history.

Key Comparisons

Feature	OST File	PST File
Purpose	Offline cache for Exchange accounts with server synchronization	Personal archive for local email storage without server link
Synchronization	Two-way sync with Exchange server; changes replicate both directions	One-way; manual creation, no automatic server synchronization
Account Requirement	Requires active Exchange Server account; created automatically	Works with any email account; manually created by user
Data Loss Risk	Low; changes sync to server automatically for backup protection	High; local file is sole copy, corruption means permanent data loss
File Size	Typically 100MB to 5GB, limited by sync settings and mailbox policies	Can exceed 20GB with no built-in limits; causes Outlook performance issues

Why It Matters

OST files are crucial for productivity in modern enterprise environments where mobile work and travel are common. Employees rely on OST caching to work offline during flights, in areas with poor connectivity, or when network services are temporarily unavailable. The automatic synchronization ensures no work is lost and maintains data consistency across multiple devices and locations.

Business Continuity: Organizations using Exchange benefit from OST files because they enable continued productivity even during network outages. Employees can continue working, and their changes are automatically synced when connectivity returns, ensuring business operations are not disrupted by temporary network issues.
Data Security Considerations: OST files contain sensitive corporate data and should be protected with appropriate file system permissions and encryption. If a laptop is stolen or compromised, an unencrypted OST file could expose confidential emails and information, making device encryption and security policies essential for protecting OST data.
Performance Impact: Large OST files can affect Outlook performance, causing slower startup times and sluggish responsiveness. Organizations typically manage this through retention policies that limit how far back email history is cached, reducing file size and improving application performance on client machines.

Understanding OST files helps IT professionals and users manage their Exchange accounts effectively. Whether troubleshooting synchronization issues, recovering from corruption, or optimizing performance, knowledge of OST file mechanics is essential for maintaining a healthy, productive email environment. Modern cloud-first approaches like Microsoft 365 and Outlook on the Web continue to reduce OST reliance, but cached mode remains valuable for users requiring robust offline capabilities and secure local data access.