An OST file (Offline Storage Table) is a local cache file created by Microsoft Outlook when using Cached Exchange Mode with Exchange Server or Microsoft 365 accounts. It stores a synchronized copy of your mailbox data locally, allowing you to work with emails, calendar, and contacts even without an internet connection.

Where is my OST file located?

In Windows 10/11, OST files are stored at C:\Users\ \AppData\Local\Microsoft\Outlook\. The file is typically named after your email address, such as 'yourname@company.com.ost'. You can find the exact location in Outlook under File > Account Settings > Data Files.

Can I open an OST file directly?

No, OST files cannot be opened directly like PST files. They are encrypted and tied to the specific Outlook profile and user account that created them. To access data from an orphaned OST file, you need to convert it to PST format using specialized OST to PST converter tools.

What happens to my OST file if I lose server connection?

When you lose connection to the Exchange server or Microsoft 365, Outlook automatically switches to offline mode using your OST file. You can continue reading existing emails, composing new messages, and managing calendar items. All changes are queued and automatically synchronized when the connection is restored.

How do I fix OST synchronization issues?

Common fixes include: checking internet connectivity, restarting Outlook, running the built-in repair tool, recreating the OST file by deleting it and letting Outlook rebuild from server, or repairing the Outlook profile. For persistent issues, check Exchange server status or contact your IT administrator.

Can I back up an OST file?

While you can copy an OST file, the backup won't be directly usable because OST files are tied to specific profiles. For proper backup, export your mailbox to PST format from Outlook, or rely on server-side backups if using Exchange/Microsoft 365. The server always has the authoritative copy of your data.

What is an OST File? - Outlook Offline Storage Format Explained

📋 On This Page

What is OST? Quick Facts How OST Works Supported Accounts Cached Exchange Mode Synchronization File Location OST Limitations Convert OST to PST Troubleshooting Recreate OST File FAQ

📖 What is an OST File?

An OST file (Offline Storage Table), also called an Offline Outlook Data File, is a synchronized replica of your Exchange Server or Microsoft 365 mailbox stored locally on your computer. Outlook creates and manages this file automatically when you use Cached Exchange Mode.

Unlike manually created data files, OST files serve as a local cache that mirrors your server mailbox. This architecture enables you to access emails, compose messages, schedule meetings, and manage contacts even when disconnected from the internet, all changes sync automatically when connectivity returns.

🔄 Synchronized Data

Complete mailbox replica
Two-way sync with server
Automatic conflict resolution
Delta synchronization (changes only)

📴 Offline Capabilities

Read all cached emails
Compose and queue messages
Manage calendar appointments
Edit contacts and tasks

💡 Key Concept: Think of your OST file as a "local mirror" of your cloud mailbox. The server always holds the authoritative copy of your data, while the OST provides fast local access and offline functionality. When you reconnect, Outlook reconciles any differences automatically.

⚡ Quick Facts

File Extension	`.ost`
Full Name	Offline Storage Table (Offline Outlook Data File)
Category	Email Cache / Synchronization File
MIME Type	`application/vnd.ms-outlook`
Developed By	Microsoft Corporation
Creation Method	Automatic (by Outlook)
File Type	Binary (encrypted, profile-locked)
Max Size (Default)	50GB (same as Unicode PST)
Directly Openable	No, tied to Outlook profile
Server Dependency	Requires Exchange or Microsoft 365
Data Authority	Server is authoritative source
Sync Direction	Bidirectional (two-way)

⚙️ How OST Works

The OST file acts as an intermediary between you and your cloud mailbox, providing seamless access regardless of connectivity:

☁️

Exchange / M365

Server Mailbox
(Authoritative)

⟷

Sync

📁

OST File

Local Cache
(Your Computer)

→

👤

You

Work Offline
or Online

Online Mode

Changes sync instantly to server. You work with the most current data. OST stays updated in real-time.

Offline Mode

Work continues from cached OST data. New emails queue in Outbox. Changes stored locally until reconnection.

Reconnection

Outlook syncs automatically. Queued emails send. Server and local changes merge seamlessly.

✅ Seamless Experience: You don't need to manually switch between online and offline modes. Outlook detects connectivity changes automatically and manages the transition transparently. The status bar shows "Connected" or "Working Offline" to indicate current state.

📧 Supported Account Types

Not all email accounts create OST files. The OST mechanism is specifically designed for server-synchronized accounts:

✅

Microsoft 365

Full OST support with cloud sync. Most common modern configuration.

✅

Exchange Server

On-premises Exchange with Cached Mode enabled creates OST.

✅

Outlook.com

Personal Microsoft accounts use OST when configured in Outlook.

❌

POP3 / IMAP

Standard email protocols don't use OST, they use PST for local storage.

📊 Account Type Comparison for Local Storage

Account Type	Local File Type	Sync Behavior	Offline Access
Microsoft 365 / Exchange	OST	Two-way automatic sync	Full offline capability
Outlook.com (Hotmail)	OST	Two-way automatic sync	Full offline capability
IMAP (Gmail, Yahoo, etc.)	OST*	Limited sync (varies)	Cached items only
POP3	PST	One-way download only	Downloaded items only

*Modern Outlook versions may create OST for IMAP accounts, but functionality differs from Exchange OST.

🗄️ Cached Exchange Mode

Cached Exchange Mode is the Outlook feature that creates and manages OST files. Understanding its settings helps optimize performance:

Cache Duration Settings

Control how much email history is stored locally in your OST file:

3 Days

Minimal

1 Month

Light

3 Months

Moderate

1 Year

Standard

2 Years

Extended

All

Complete

How to Configure Cached Exchange Mode

Open Outlook and click File in the menu.
Click Account Settings → Account Settings.
Select your Exchange/M365 account and click Change.
Check "Use Cached Exchange Mode" if not already enabled.
Use the slider to select how much mail to keep offline.
Click Next/Done and restart Outlook for changes to take effect.

✅ Benefits of More Cache:

Search older emails while offline
Access complete email history
Better experience during server outages
Faster access to historical data

⚠️ Tradeoffs of More Cache:

Larger OST file size
Longer initial sync time
More disk space required
Potentially slower Outlook startup

🔄 Synchronization Process

Understanding how OST synchronization works helps diagnose issues and set realistic expectations:

Sync Timeline Example

Outlook Starts

Connects to Exchange/M365 server, checks for changes since last sync.

Delta Sync Begins

Downloads only new/changed items, not the entire mailbox. Efficient and fast.

Folder Sync

Each folder syncs based on priority: Inbox first, then Sent, Calendar, others.

Connection Lost

Outlook switches to offline mode. You continue working from OST cache.

Reconnection

Connection restored. Local changes upload, server changes download. Conflicts resolved.

📥 Server → OST (Download)

New emails received on server
Calendar invites from other users
Shared folder updates
Changes made via webmail or mobile
Deletions performed elsewhere

📤 OST → Server (Upload)

Sent emails from Outbox
Calendar appointments you create
Contact edits you make
Read/unread status changes
Folder organization changes

🔍 Sync Status: Check synchronization status in Outlook's status bar (bottom). Look for "All folders are up to date" or folder-specific sync progress. You can also press Ctrl+Shift+F6 or check Send/Receive → Send/Receive Groups → View Progress.

📍 OST File Location

OST files are stored in a fixed location determined by Windows and Outlook. Unlike PST files, you cannot choose where to save them:

Default OST Location (Windows 10/11):

C:\Users\<YourUsername>\AppData\Local\Microsoft\Outlook\

Note: The AppData folder is hidden by default. Enable "Show hidden files" in File Explorer, or type %localappdata%\Microsoft\Outlook in the address bar.

OST File Naming Convention

OST files are named based on your email account:

yourname@company.com.ost, Standard format
yourname@company.com - backup.ost, If original existed
Outlook Data File.ost, Generic name (older versions)

Finding Your OST File in Outlook

Method 1: Account Settings

File → Account Settings → Account Settings
Click the Data Files tab
Select your account
Click Open File Location

Method 2: Hold Ctrl + Right-Click

Hold Ctrl key
Right-click Outlook icon in system tray
Select Connection Status
View file path in dialog

⚠️ Cannot Move OST Files: Unlike PST files, OST files must remain in their default location. Moving an OST file will break the Outlook profile connection. If you need to relocate Outlook data, consider using Symbolic Links (advanced) or simply let Outlook recreate the OST in the new profile.

⚠️ OST Limitations

While OST files provide excellent offline functionality, they have important limitations to understand:

🚫 What You CAN'T Do

Open directly: Cannot double-click to open like PST
Transfer between computers: Profile-locked encryption
Use as backup: Tied to specific Outlook profile
Open in other email clients: Outlook-only format
Access if account removed: Becomes orphaned
Move to different location: Must stay in AppData

✅ What You CAN Do

Work offline: Full email, calendar, contacts access
Queue emails: Send when connection returns
Search cached mail: Fast local search
Convert to PST: Using third-party tools
Recreate if corrupted: Server has authoritative data
Control cache size: Adjust sync period

🚨 Critical Understanding: The OST file is NOT a backup. It's a cache that depends on the server. If your Exchange account is deleted or you leave an organization, your OST becomes inaccessible. Always export important data to PST format for true portability and backup.

🔄 Convert OST to PST

Converting OST to PST is essential when you need portable access to your data, especially if you're leaving an organization or need a true backup:

✅ Method 1: Export from Outlook (Recommended)

Use this if you still have access to the Exchange/M365 account:

Open Outlook with your account connected
Click File → Open & Export → Import/Export
Select "Export to a file" → Next
Choose "Outlook Data File (.pst)" → Next
Select your mailbox root folder, check "Include subfolders"
Choose save location and set optional password
Click Finish and wait for export to complete

⚠️ Method 2: OST Converter Tools

Use this for orphaned OST files (account no longer accessible):

Download a reputable OST to PST converter
Install and launch the conversion tool
Browse to your OST file location
Select the OST file to convert
Choose PST as output format
Select destination folder for new PST
Start conversion and wait for completion

Note: Third-party tools vary in quality. Some free tools have limitations.

When to Convert OST to PST

🏢

Leaving Job

Export before losing account access

💾

Backup Needs

Create portable backup copy

🔀

Migration

Moving to different email service

⚖️

Legal Hold

Preserve emails for compliance

🔧 Troubleshooting

Common OST-related issues and their solutions:

Check connection: Verify internet connectivity and server availability
Manual sync: Press F9 or click Send/Receive All Folders
Check status: Look at status bar for sync errors
Server issues: Check with IT if Exchange/M365 is experiencing problems
Cached Mode: Ensure Cached Exchange Mode is enabled in account settings

Close Outlook: Ensure no Outlook processes running (check Task Manager)
End OUTLOOK.EXE: Kill any stuck Outlook processes
Antivirus: Temporarily disable or add OST folder to exclusions
Restart computer: Clear all file locks
Check sync tools: Third-party sync apps may lock the file

Wait it out: Large mailboxes take time, especially first sync
Reduce cache: Lower the sync period (e.g., 3 months instead of All)
Disable add-ins: Start Outlook in Safe Mode (hold Ctrl while launching)
Recreate OST: Delete OST file and let Outlook rebuild (see below)
Check disk space: Ensure sufficient free space for OST file

Corrupted OST: Rename/delete OST file and let Outlook recreate
Profile issue: Create new Outlook profile (Control Panel → Mail)
Repair Office: Use Programs & Features → Microsoft Office → Repair
Credential issues: Clear stored passwords in Credential Manager

Reduce cache period: Change from "All" to 1 year or less
Empty Deleted Items: Large deleted items consume space
Clear Junk folder: Spam accumulates over time
Compact mailbox: File → Account Settings → Data Files → Compact (may not reduce OST much)
Archive old items: Move to PST file instead of keeping in OST

🔄 Recreate OST File

If your OST file is corrupted or causing issues, the safest solution is to delete it and let Outlook create a fresh copy from the server:

✅ Safe to Delete: Since the Exchange/M365 server holds the authoritative copy of all your data, deleting the OST file only removes the local cache. Your emails, contacts, and calendar items are safe on the server and will re-download.

Step-by-Step: Recreating Your OST File

Close Outlook Completely
Exit Outlook and verify in Task Manager that OUTLOOK.EXE is not running.
Navigate to OST Location
Open File Explorer, type %localappdata%\Microsoft\Outlook in address bar, press Enter.
Find Your OST File
Locate the file named after your email (e.g., yourname@company.com.ost).
Rename or Delete the OST
Rename to .ost.old (safer) or delete the file. Renaming lets you recover if needed.
Restart Outlook
Open Outlook. It will create a new OST file and begin synchronizing from the server.
Wait for Sync to Complete
Initial sync may take minutes to hours depending on mailbox size. Status bar shows progress.

⚠️ Sync Time Warning: Recreating an OST for a large mailbox can take considerable time. A 10GB mailbox might take 30-60 minutes or more depending on connection speed. Plan accordingly and avoid doing this before important meetings.

❓ Frequently Asked Questions

You cannot transfer an OST file to a new computer. When you set up Outlook on a new PC:

Add your Exchange/M365 account to Outlook
Outlook creates a brand new OST file
Your mailbox data syncs from the server automatically
The old OST on your previous computer becomes useless

Pro tip: If you have local-only data, export to PST before switching computers.

Yes, but it requires third-party conversion tools:

Download a reputable OST to PST converter (free and paid options exist)
Open the orphaned OST file with the converter
Export contents to PST format
Open the resulting PST in any Outlook installation

Note: Success rate varies depending on OST file integrity and the tool used.

OST files can be larger than server mailbox size due to:

Deleted item retention: Items in Deleted Items still occupy space
White space: Deleted content leaves gaps that aren't immediately reclaimed
Metadata: Local indexes and caching information
Attachments: May be stored in expanded form locally

Solution: Reduce cache period and empty Deleted Items folder regularly.

Yes! The OST is a two-way synchronized cache:

Deleting an email in Outlook deletes it locally AND syncs the deletion to server
This is intentional, OST mirrors your server mailbox
Deleted items go to Deleted Items folder (both locally and on server)
Permanent deletion affects both locations

Exception: Items deleted while offline will sync the deletion when you reconnect.

Yes, by disabling Cached Exchange Mode:

Go to File → Account Settings → Account Settings
Select your account → Change
Uncheck "Use Cached Exchange Mode"
Outlook will work directly with the server (Online Mode)

Tradeoff: No offline access, slower performance, every action requires server connection.

Microsoft's ScanPST.exe can technically scan OST files, but it's not recommended:

Better approach: Delete the OST and let Outlook recreate it from server
Why: Server has authoritative data, so rebuilding is cleaner than repairing
When to use ScanPST on OST: Only if you cannot connect to server and need local data
Third-party tools: Some specialize in OST repair for orphaned files

Only when necessary, there's no regular schedule needed:

Recreate when: Experiencing sync issues, corruption errors, or major slowdowns
Don't recreate routinely: It causes full re-download and wastes time
After major changes: Some admins recommend after mailbox migrations
Typical frequency: Most users never need to do this manually