Repair OST File: Complete Guide [Free & Pro Fixes 2025]

An OST file, or Offline Storage Table, is a data file used by Microsoft Outlook to store a local cache of your mailbox data. It enables you to access emails, contacts, calendars, and other mailbox items even without an Internet connection. When you’re online, Outlook automatically synchronizes the OST file with the Exchange Server or IMAP account, ensuring your data stays up-to-date across all devices.

OST files are created when you configure Outlook with Exchange, Office 365, IMAP, or Outlook.com accounts using Cached Exchange Mode. Any changes you make offline are saved to the OST file and synchronized with the server once your connection is restored.

The key difference between OST and PST files is that OST files work with server-based accounts and synchronize automatically, while PST files are standalone archive files that don’t sync with any server.

1.2 Signs Your OST File is Corrupted

Recognizing the symptoms of OST file corruption helps you take prompt action to repair the file before data loss occurs. Common signs include:

Error messages stating “Unable to open your default e-mail folders” or “The file xxx.ost is in use and cannot be accessed”.
Synchronization failures between Outlook and Exchange Server
Missing emails, folders, or other mailbox items
Outlook crashes, freezes, or hangs when accessing the OST file
Slow performance or delayed response when working with emails

1.3 Common Causes of OST File Corruption

Understanding why OST files become corrupt helps you prevent future issues. The main causes include:

Hardware-Related Issues: sudden power failure, hard drive failures and bad sectors, storage device problems, hardware compatibility issues
Software-Related Issues: improper Outlook termination, Outlook software bugs or conflicts, outdated Outlook versions, incompatible add-ins, antivirus programs, etc.
File Size Issues: oversized OST files, size limit problems
External Factors: virus or malware attacks, network connectivity issues, synchronization failures, human error such as accidental deletion or file moving

2. Locate Your Outlook OST File

Before you can repair an OST file, you need to know where it’s stored on your computer. Outlook provides several methods to locate your OST file.

2.1 Using Outlook to Find OST Location

The easiest way to find your OST file location is through Outlook’s Account Settings:

Open Microsoft Outlook
Click File -> Account Settings -> Account Settings
Select the Data Files tab
Select the OST file you want to locate
Click Open File Location to view the folder containing your OST file

2.2 Common OST File Locations

For Windows 7 and higher versions, OST files are typically stored in the following default location:

C:\Users\[Username]\AppData\Local\Microsoft\Outlook

Replace [Username] with your actual Windows username. The OST file is usually named after your email address, such as yourname@company.com.ost.

2.3 Finding OST Files Manually

If you need to locate OST files manually, use Windows File Explorer:

Press Windows + E to open File Explorer
In the search box, type *.ost to search for all OST files
Enable viewing hidden files by clicking View -> Hidden items
Navigate to the AppData folder if the search doesn’t find your files

3. Method 1: Close Outlook and Reopen

Sometimes, OST file errors are temporary and can be resolved by simply closing and reopening Outlook:

Go to File -> Exit to close Outlook properly
Ensure no Outlook processes are running in Task Manager
Wait 30 seconds before reopening Outlook
Launch Outlook and check if the issue persists

4. Method 2: Disable and Re-enable Cached Exchange Mode

Cached Exchange Mode allows Outlook to work offline with a copy of your mailbox. Disabling and re-enabling this feature can fix OST file synchronization problems and sometimes resolve corruption issues.

Important Limitation: This method only works with Exchange accounts. It is not available for IMAP, POP3, or Outlook.com accounts.

Open Outlook
Go to File -> Account Settings -> Account Settings
Select your account
Click Change
Uncheck Use Cached Exchange Mode
Click Next and then Finish
Restart Outlook.
Repeat steps 1-6 and re-enable Use Cached Exchange Mode
Restart Outlook let it synchronize with the server

When you re-enable Cached Exchange Mode, Outlook creates a fresh OST file and downloads your mailbox data from the server. This process may take time, depending on your mailbox size.

5. Method 3: Clear Offline Items and Update Folder

If corruption affects specific folders rather than the entire OST file, you can clear those folders and then re-sync the items manually.

Important Limitation: This method only works with Exchange accounts. It is not available for IMAP, POP3, or Outlook.com accounts.

Open Outlook and right-click the affected folder
Click Properties
In the Properties dialog box, click Clear Offline Items
Click OK to confirm
Go to the Send/Receive tab
Click Update Folder to synchronize the folder with the server

6. Method 4: Compact OST File

Sometimes compacting the OST file can fix corrupt OST file, especially when the corruption is caused by an oversized problem.

Open Outlook and right-click the affected OST file.
Click Data File Properties
In the Properties dialog box, click Advanced
Click Compact Now to compact the OST file.

7. Method 5: Delete and Recreate OST File

When the OST file is severely corrupted, deleting it will allow Outlook to recreate and resync a fresh copy of the OST file from the server, which can resolve the issue.

Find the OST file location.
Close Outlook completely.
Right-click the OST file and select Rename
Add .bak to the end of the filename (e.g., yourname@company.com.ost.bak)
Reopen Outlook
Outlook will automatically create a new OST file and begin resynchronizing with the server

Limitations:

Requires an active connection to the Exchange Server
Any changes not synchronized with the server will be lost

8. Method 6: Recreate a New Outlook Profile and Re-sync OST File

If the Outlook profile itself is corrupted, creating a new profile can resolve both profile and OST file issues.

Close Outlook and ensure no Outlook processes are running
Go to Control Panel -> Mail
Click Show Profiles
Click Add to create a new profile
Enter a name for the new profile and click OK
Follow the prompts to configure your email account
Enter your email address and password when prompted
Click Next and wait for Outlook to configure the account
Select the new profile as your default profile
Click OK to close the Mail Setup dialog box
Open Outlook with the new profile
Outlook will create a new OST file and synchronize your mailbox data

This method has the same limitations as Method 4: it requires an Exchange Server connection, and any changes not synced to the server will be lost.

9. Method 7: Use the Inbox Repair Tool (ScanPST.exe)

The Inbox Repair Tool, also known as ScanPST.exe, is Microsoft’s built-in utility for diagnosing and repairing errors in Outlook data files. While primarily designed for PST files, it can also scan and fix certain types of OST file corruption.

You can refer to our comprehensive guide on how to use ScanPST.exe.

10. Method 8: Use OST Integrity Check Tool (ScanOST.exe)

ScanOST.exe is the OST Integrity Check Tool designed specifically for OST files in older Outlook versions. It scans for inconsistencies between the local OST file and the Exchange Server, then repairs synchronization errors.

Important Note: ScanOST.exe is only available in Outlook 2007 and earlier versions. For Outlook 2010 and later, use ScanPST.exe instead.

Close Outlook completely
Navigate to C:\Program Files\Microsoft Office\Office[version]
Double-click SCANOST.EXE to launch the tool
Select the Outlook profile that contains the corrupt OST file
Choose either Scan all folders or Choose folder to scan
Make sure Repair errors is selected
Click Begin Scan to start the process
Wait for the scan and repair to complete
Open Outlook and verify the OST file is accessible

11. Method 9: Check Disk Errors

Hard drive errors or bad sectors can cause OST file corruption. Running Windows Disk Check utility can identify and fix these storage device issues, potentially resolving OST file problems.

Close all programs including Outlook
Press Windows + E to open File Explorer
Right-click on the drive containing your OST file (usually C:)
Select Properties
Click the Tools tab
Under Error checking, click Check
Click Scan drive to begin the disk scan
If errors are found, follow the prompts to repair them
Restart your computer if prompted
After the scan completes, open Outlook and test the OST file

You can also run the Check Disk utility from Command Prompt for more advanced options:

Press Windows + X and select Terminal (Admin)
Type chkdsk C: /f /r and press Enter
If prompted to schedule the check on next restart, type Y and press Enter
Restart your computer to run the disk check

12. Method 10: Use Windows System Restore

If OST file corruption occurred after a recent system change, software installation, or Windows update, System Restore can revert your computer to a previous state when the OST file was functioning correctly.

Close all programs, including Outlook
Press Windows + R to open the Run dialog
Type rstrui.exe and press Enter
Click Next in the System Restore window
Select a restore point from before the OST corruption began
Click Next to continue
Review the restore point details and click Finish
Click Yes to confirm the system restore
Wait for the restore process to complete (your computer will restart)
After restart, open Outlook and check if the OST file issue is resolved

13. Method 11: Third-Party OST Repair Tools

When manual methods fail or the OST file corruption is too severe, third-party OST repair tools offer advanced recovery capabilities. These specialized tools can handle extensive corruption, recover data from orphaned OST files, and convert OST files to PST format for easy import into Outlook.

Below are the steps to use one of such tools, DataNumen Exchange Recovery, to recover OST file:

Start DataNumen Exchange Recovery.
Select the OST file to be recovered.
Set the output PST file name.
Click Start Recovery
After the recovery process, open the output PST file in Outlook, which contains all the data in the corrupt OST file.

14. Fixes for Persistent OST File Corruption

If OST file corruption persists after you recreate it, the corruption may be caused by underlying Outlook or system problems. You can try the following solutions.

14.1 Update Outlook

Outdated Outlook versions may contain bugs that cause OST file corruption. Installing the latest updates ensures you have all bug fixes and performance improvements.

Open Outlook
Click File -> Office Account
Click Update Options
Select Update Now
Wait for the updates to download and install
Restart Outlook when prompted
Verify the OST file functions properly after updating

14.2 Disable Incompatible Add-ins

Faulty or incompatible Outlook add-ins can interfere with OST file operations and cause corruption. Disabling add-ins helps identify and remove problematic extensions.

Start Outlook in safe mode by pressing Windows + R, typing Outlook.exe /safe, and pressing Enter
Go to File -> Options
Click Add-ins in the left panel
At the bottom, select COM Add-ins from the Manage dropdown
Click Go
Uncheck all add-ins to disable them
Click OK
Restart Outlook normally and test the OST file
If the issue is resolved, enable add-ins one by one to identify the problematic one
Remove or update the incompatible add-in

14.3 Repair Office Installation

Corrupted Office installation files can cause persistent Outlook and OST file problems. Repairing your Office installation can fix these underlying issues.

Close all Office applications including Outlook
Open Control Panel -> Programs -> Programs and Features
Find Microsoft Office or your specific Office version
Right-click the application and select Change
Choose Quick Repair first and click Repair
If Quick Repair doesn’t work, repeat the process and choose Online Repair
Follow the prompts and restart your computer when prompted
Open Outlook and verify OST file functionality

14.4 Use System File Checker

Corrupted system files can affect Outlook’s ability to access and manage OST files properly. The System File Checker (SFC) tool automatically repairs any corrupted system files it finds, which may resolve persistent OST file corruption issues caused by system-level problems.

Close all programs, including Outlook
Press Windows + X and select Terminal (Admin) or Windows PowerShell (Admin)
Type sfc /scannow and press Enter
Wait for the scan to complete (this may take 15-30 minutes)
Review the results for any repaired files
Restart your computer
Open Outlook and test the OST file

15. Best Practices to Prevent OST File Corruption

Preventing OST file corruption is more effective than repairing it. Following these best practices helps maintain OST file integrity and reduces the risk of data loss.

15.1 Regular Maintenance

Keep Outlook updated with the latest patches and updates
Monitor OST file size to prevent exceeding limits
Regularly archive old items to separate PST files
Split large OST files when approaching size limits

15.2 Proper Outlook Usage

Always close Outlook properly using the Exit command
Avoid forcing Outlook to close through Task Manager
Exit Outlook before shutting down your computer

15.3 System Security

Maintain up-to-date antivirus protection
Enable real-time malware prevention
Perform regular security scans on your system

15.4 Network Considerations

Ensure stable internet connection during synchronization
Use reliable network connections when syncing large mailboxes
Maintain stable VPN connections if required for Exchange access

15.5 Hardware Maintenance

Perform regular hard drive health checks
Use an uninterruptible power supply (UPS) to protect against power failures
Run disk maintenance utilities periodically

15.6 Backup Strategies

Create regular backups of OST files
Convert important OST data to PST format for archiving
Store backup copies in multiple locations
Implement automated backup solutions

15.7 Add-in Management

Only use compatible and trusted add-ins
Regularly review installed add-ins and remove unused ones
Disable unnecessary add-ins to improve stability

16. Frequently Asked Questions (FAQs)

Q: What is the difference between OST and PST files?

A: OST files are Offline Storage Table files that work with server-based accounts like Exchange and Office 365. They synchronize automatically with the server, allowing offline access to your mailbox. PST files are Personal Storage Table files that serve as standalone archives and do not sync with any server. OST files require an active account connection to function, while PST files can be used independently.

Q: Can I open an OST file without Outlook?

A: OST files cannot be opened directly without Outlook because they are designed specifically for use with Outlook’s Cached Exchange Mode. However, you can use third-party OST to PST converter tools to extract the data and convert it to PST format, which can then be imported into Outlook or viewed with other email clients that support PST files.

Q: How do I know if my OST file is corrupted?

A: Signs of a corrupt OST file include error messages when opening Outlook, synchronization failures with the Exchange Server, missing emails or folders, frequent Outlook crashes or freezes, slow performance when accessing emails, and error messages stating the OST file cannot be opened. If you experience any of these symptoms, your OST file may be corrupted.

Q: Can ScanPST.exe repair all OST file corruption?

A: No, ScanPST.exe has limitations. It works well for minor to moderate corruption but cannot repair severely damaged files, files larger than 2GB in older Outlook versions, encrypted or password-protected files, or orphaned OST files without server access. For extensive corruption, third-party OST repair tools are more effective.

Q: Will I lose data when repairing OST files?

A: The risk of data loss depends on the repair method used. Manual methods like recreating the OST file or creating a new profile will only preserve data that has been synchronized with the Exchange Server. Any offline changes not yet synced will be lost. ScanPST.exe creates a backup before repair, minimizing data loss risk. Third-party tools generally offer the safest recovery with minimal data loss.

Q: How long does it take to repair a corrupt OST file?

A: Repair time varies based on the file size, corruption severity, and method used. ScanPST.exe typically takes 5-30 minutes for moderate-sized files. Recreating an OST file by syncing with the server can take several hours for large mailboxes. Third-party tools usually complete repairs in 10-60 minutes depending on file size and corruption level.

Q: Can I recover deleted emails from a corrupt OST file?

A: Recovery of deleted emails depends on whether they still exist on the Exchange Server. If deleted items are still in the server’s Deleted Items folder or recoverable items, recreating the OST file will restore them. Manual methods generally cannot recover permanently deleted items. Third-party OST repair tools offer the best chance of recovering deleted items that remain in the corrupt OST file.

Q: What is the maximum size limit for OST files?

A: OST file size limits vary by Outlook version. Outlook 2010 and later support OST files up to 50GB by default, though this can be increased via registry settings. Older Outlook versions (2007 and earlier) had a 2GB limit for ANSI files and 20GB for Unicode files. Exceeding these limits can cause performance issues and corruption.

Q: Can OST files become oversized and affect performance?

A: Yes, oversized OST files significantly impact Outlook performance. Large files cause slow startup, delayed email loading, sluggish search, and increased risk of corruption. To maintain performance, regularly archive old items, delete unnecessary emails, and keep your OST file size manageable. Consider splitting large mailboxes into multiple PST archives.

Q: Is it safe to delete OST files?

A: Yes, it is generally safe to delete an OST file because it contains a synchronized copy of data stored on the Exchange Server. When you delete and recreate the OST file, Outlook downloads all mailbox data from the server. However, any offline changes not yet synchronized will be lost. Always ensure synchronization is complete before deleting an OST file.

Q: Can I transfer OST files to another computer?

A: You can physically transfer OST files to another computer, but they cannot be used directly. OST files are tied to the specific Outlook profile and computer where they were created. To access your mailbox on another computer, configure Outlook with your email account, and it will create a new OST file and sync with the server. Alternatively, convert the OST to PST format for portability.

Q: Are there free OST repair tools available?

A: Microsoft provides ScanPST.exe and ScanOST.exe as free built-in repair tools. However, they have limitations with severely corrupted files. Most third-party OST repair tools offer free trial versions that allow you to scan and preview recoverable data, but full recovery requires purchasing the software. The trials help evaluate whether the tool can recover your specific data before investing.

Q: Can I repair OST files on Mac?

A: Most OST repair tools are designed for Windows systems. To repair OST files on Mac, you can transfer the OST file to a Windows computer or virtual machine, use a Windows-compatible repair tool, then transfer the repaired or converted PST file back to Mac. Some third-party tools may offer Mac-compatible versions, but these are less common than Windows solutions.

Q: What should I do if the Inbox Repair Tool is not responding?

A: If ScanPST.exe stops responding, it usually indicates an oversized or severely corrupted file. Try closing the tool and running it again. If the problem persists, consider splitting the repair process by converting the OST to PST first using a third-party tool, or use an alternative OST repair utility designed to handle complex corruption. Ensure no other programs are accessing the OST file during repair.

Q: How can I prevent OST file corruption in the future?

A: Prevent corruption by keeping Outlook updated, closing it properly before shutdown, maintaining stable network connections during sync, using antivirus protection, avoiding oversized OST files through regular archiving, disabling incompatible add-ins, ensuring hard drive health, and creating regular backups. Following these practices significantly reduces corruption risk and protects your mailbox data.

17. Conclusion

17.1 Summary of Repair Methods

Repairing corrupt OST files requires understanding both the symptoms and the appropriate solutions. This guide covered multiple approaches, from simple fixes like reopening Outlook to advanced methods using professional repair tools. Manual methods work well for minor corruption and synchronization issues, while third-party tools excel at recovering severely damaged files and orphaned OST data.

The best approach depends on your specific situation. Start with simple methods like disabling Cached Exchange Mode or using ScanPST.exe. If these fail, consider recreating the OST file or creating a new Outlook profile. For severe corruption or orphaned files without server access, professional OST repair tools offer the most reliable recovery solution.

17.2 Key Takeaways

Regular backups are essential for protecting against data loss from OST file corruption. Prevention through proper maintenance, timely updates, and careful Outlook usage is more effective than repairing corruption after it occurs. Understanding the different repair methods helps you choose the right solution based on corruption severity and your specific needs.

When manual methods fail, don’t hesitate to use professional OST repair tools. These specialized solutions can recover data that built-in utilities cannot, preserving your critical emails and mailbox items. Always verify that your data is synchronized with the Exchange Server before attempting repairs that involve deleting or recreating OST files.

17.3 Final Recommendations

Implement regular maintenance practices to keep your OST files healthy. Archive old emails periodically, monitor file size, keep Outlook updated, and ensure stable network connections during synchronization. Use antivirus software to prevent malware-related corruption, and always close Outlook properly before shutting down your computer.

Create a backup strategy that includes regular OST file backups and conversion to PST format for long-term storage. Consider using automated backup solutions to ensure consistent data protection. By following these proactive measures and knowing how to repair OST files when corruption occurs, you can maintain reliable access to your Outlook data and minimize the impact of file corruption on your productivity.

References

About the Author

Xiu Xing is a seasoned IT professional with over 8 years of experience in enterprise email systems and Microsoft Office troubleshooting. He has helped thousands of users resolve complex Outlook issues across diverse corporate environments.

Xiu specializes in email system migrations, data recovery, and productivity software optimization. His hands-on experience includes managing Exchange Server deployments, troubleshooting MAPI connectivity issues, and implementing robust backup strategies for mission-critical email systems.

Through his technical writing, Xiu is committed to making complex IT solutions accessible to everyday users. He continuously stays current with the latest Office 365 and Exchange Online developments and emerging email technologies. When not solving email mysteries, Xiu enjoys sharing practical tech tips that help people work more efficiently.

Have questions about this guide or need additional help with Outlook issues? Xiu welcomes feedback and suggestions for improving these troubleshooting resources.

How to Repair Corrupt OST File: Complete Guide [2025]

1. Introduction

1.1 What is an OST File?