Microsoft Outlook depends on OST files to store synchronized mailbox data from Exchange Server, Microsoft 365, and IMAP accounts. This file allows users to access data like emails, calendar, contacts, and other mailbox items even when working offline.

Many users encounter these errors:

OST is in use and cannot be accessed. Close any application that is using this file, and then try again. You might need to restart your computer.

When these errors appear, Outlook may fail to start, synchronization may stop working, and user may lose access to their mailbox data. But we can resolve “OST is in use and cannot be accessed” issue by understanding the root cause of this problem to apply correct approach.

What Does “Outlook OST Cannot be Accessed ” Mean?

This error indicates that your Outlook is trying to open your OST file, but it’s currently locked by another application, background process, synchronization service, or due to corruption. So, Outlook can’t access the data. As a result, Outlook starts showing errors such as:

Common Symptoms

  • Outlook fails to open.
  • Send/Receive operations stop working.
  • Synchronization errors appear repeatedly.
  • Outlook freezes during startup.
  • Email folders fail to update.
  • Exchange or Microsoft 365 mailbox becomes inaccessible.
  • OST-related error messages appear in Outlook logs.

Main Causes of “OST is In Use and Cannot be Accessed” Error

  • Multiple Outlook Instances Running: This is common when Outlook processes continue running in the background even after closing the application. When new session starts, it can’t access the OST file because another process is still holding it.
  • Skype for Business or Microsoft Lync Integration: Applications such as Skype for Business and Microsoft Lync use Outlook data for conversation history, presence status, contacts synchronization, and calendar integration. If these applications lock the OST file during synchronization.
  • Antivirus Software Scanning OST Files: Many antivirus programs temporarily lock the OST file during scanning operations, which causes access issue.
  • Synchronization Conflicts: Large mailboxes, unstable internet connections, or interrupted Exchange synchronization can leave the OST file in an inconsistent state.
  • File Permission Problems: if your Windows profile permissions are damaged, OST file copied from another system, or security settings were modified can make your file inaccessible.
  • Third-Party Outlook Add-ins: Some Outlook add-ins can affect and interfere with OST operations, which may lock your file and shows “OST is in use and cannot be accessed” error.

After understanding the root cause, let’s move to the troubleshooting techniques.

How to Fix “OST File is in Use and Cannot Be Accessed”?

Before applying any advanced troubleshooting methods, we will start with the simplest solution. We have explained all techniques below one by one:

Method 1: Close All Outlook Processes

This is the basic and right way to fix “OST is in use and cannot be accessed” error by closing application through Task Manager.

  1. Open “Task Manager” using “Ctrl + Shift + Esc” keys.
  2. Locate Outlook application.
  3. Select each process and right-click.
  4. And choose “End Task” option.
  5. Finally, restart your Outlook.

Method 2: Restart the Computer

Restarting the system can release locked files and allow access to OST file.

After rebooting, start Outlook again and check whether it is showing error.

Method 3: Fix “OST is in Use and Cannot be Accessed” by Starting Outlook in Safe Mode

Safe mode helps Outlook to start without third-party add-ins, if your application opens successfully, then remove all the problematic third-party add-ins.

  1. Use “Windows + R” key to open Run.
  2. And type: Outlook.exe /safe.
  3. Hit the “Enter” button.

Method 4: Create a New Outlook Profile

Damaged Outlook profile is another reason for “OST is in use and cannot be accessed” error. Recreating it can resolve this issue.

  1. Open your “Control Panel”.
  2. Choose “Mail” from here.
  3. Select the “Show Profiles” >> “Add” option.
  4. Create a new profile.
  5. Configure the email account.
  6. And set up new profile as default.

Now Outlook will create fresh OST file automatically.

Method 5: Fix the “Outlook OST Cannot be Accessed” by Rebuilding OST File

Rebuilding is often most effective solution to fix “OST is in use and cannot be accessed” issue.

  1. Close your Outlook completely.
  2. Now navigate to: C:\Users\Username\AppData\Local\Microsoft\Outlook
  3. Locate your OST file.
  4. Rename it and restart Outlook.

Important Note: Before choosing this technique, make sure all your mailbox data exists on the Exchange Server or Microsoft 365 account. Because unsynchronized offline data may be lost.

Method 6: Check Disk Error

Hard drive issues can also affect your OST file, which may cause “Outlook OST cannot be accessed” error.

Run CHKDSK

Open Command Prompt as Administrator and execute:

chkdsk/f/r

And restart the system if prompted.

This technique will scan and repair file system errors, which may prevent Outlook from accessing OST file.

What If Your OST File is Damaged?

If nothing works then verify your data file, because damaged or corrupted OST file is the biggest and another reason of “OST is in use and cannot be accessed” error. This can happen due to unexpected system shutdowns, Outlook crashes, disk error, malware attacks or synchronization interruptions.

Common Signs of OST Corruption Include:

  • Outlook fails to start or freezes frequently.
  • Send/Receive operations do not complete.
  • Mailbox folders appear empty or inaccessible.
  • Synchronization errors occur repeatedly.
  • Outlook displays OST-related error messages.
  • Missing emails, contacts, calendars, or attachments.

If your mailbox data is available on Exchange Server or Microsoft 365, then Outlook can typically create a new OST file or download the mailbox data again. However, if it contains unsynchronized offline data or belongs to an inaccessible mailbox then recreating file may lose your important data.

Not only this, but damaged file can also cause “OST is in use and cannot be accessed” error. In such cases, using a professional tool is required. Because recovering data from damaged file is not possible manually. Download SysTools OST Recovery Tool.

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Best Practices to Prevent OST Access Errors

To prevent from “Outlook OST cannot be accessed” error in future, follow these simple tricks:

  • Keep your Outlook updated.
  • Avoid sudden system shutdown.
  • Maintain stable network connectivity.
  • Analyze your mailbox size regularly.
  • Remove unnecessary third-party add-ins.
  • Create regular mailbox backups.

Conclusion

The “OST is in use and cannot be accessed” error usually occurs due to another process locking your OST file, synchronization conflicts, or a damaged OST file. In most cases, restarting Outlook, disabling conflicting applications, rebuilding Outlook profile or OST file can resolve this problem.

But for severely damaged and orphaned OST file, use professional solution because it can recover complete mailbox including deleted data into healthy format. You can connect with us through live chat support.