Error Solved: “The file username.ost is in use and cannot be accessed” in Outlook

Summary: OST is an Outlook data file and problems with OST file often results in errors such as ‘The file xxx.ost is in use and cannot be accessed’. This can be resolved by converting the faulty OST file to PST format with Stellar Converter for OST. However, there are a few troubleshooting methods that you may apply to fix the Outlook error.

In Microsoft Exchange, if Cached Exchange Mode is ON, a replica of mailboxes residing on the server is created on the local machine with .ost extension. This ‘cached mode’ allows users to work offline on Outlook and improves Outlook performance.

When the server comes back online, all changes made in the local mailbox cache (.OST) are synchronized with the mailbox on the server. However, sometimes due to a problem with OST file, an Outlook user may experience errors such as,

‘The file C:\users\….username.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 computer.’

Reasons behind the Error

Tip: When “Ost is in use and cannot be accessed” error occurs or OST file becomes inaccessible, you can try to make the file accessible again by using manual methods, but they may cause damage to the OST file. We recommend using Stellar Converter for OST software to avoid any damage to the inaccessible OST file by converting it into a PST file while maintaining data integrity. 

The error may occur due to several issues but often appears when a user tries to “Share” or “Save and Send” MS Office file via applications such as MS Word, PowerPoint, Excel etc.

The error indicates that the Outlook OST file is being used by another program and closing it may resolve the error. However, spotting the particular program or process which is causing the error can be difficult for most users. The error also suggests user to restart the system which usually resolves the error.

But it isn’t necessary that the OST file is being used by another program. There could be several other reasons which may lead to the error, such as:

  • Synchronization issue between the OST and associated Exchange mailbox
  • Conflicting applications with Microsoft Outlook
  • Multiple users trying to access the same OST file (typically in case of networked systems)
  • Unreliable network connectivity or slow internet connection
  • Unauthorized access to OST file
  • Processes like Office Communicator 2005/2007, LYNC, UCMAPI.EXE etc. running in the background

How to Resolve the Error in MS Outlook 2019, 2016, 2013, 2010 & 2007?

Following are a few methods that can help you troubleshoot and fix “The file xxx.OST is in use and cannot be accessed” error:

Method 1: Quit MS Outlook and Related Processes

Make sure that multiple instances of Outlook aren’t running on the same machine and no other related process is running which need access to the OST file. This can cause the OST file to be marked as “in use”. Here are the steps:

  • Close Outlook and open Task Manager
  • Select the “Processes” tab
  • In the list, look for “Outlook”, “LYNC”, “Communicator”, or “UCMAPI” named processes

  • Click on such processes and hit “End Task” button one-by-one

Then check if the error is resolved.

Method 2: Disable Exchange Cached Mode 

Another way to fix the ‘OST can’t be accessed’ issue is to turn OFF the “Cached Mode”. Follow these steps,

  • Quit Outlook and then open Control Panel
  • Go to Mail> Email Accounts> Email
  • Select your email-id and then click on the “Change” button
  • Uncheck “Use Cached Exchange Mode…

  • Click on the Next button
  • Click on Finish

Don’t forget to turn Cached Mode ON even if the error isn’t fixed. 

TIP: If the OST is stored on Network drive, changing its location to a local drive may also fix the error. 

Method 3: Find and Disable Problematic Outlook Add-ins

Though add-ins are meant to enhance Outlook’s functionality, faulty add-ins can sometimes cause trouble such as making OST file inaccessible.

Thus, disabling the problematic add-ins may fix this error and you may access the Outlook account and OST file. To find if the error is a result of a faulty add-in, you need to start Outlook in Safe Mode. Follow these steps,

  • Close Outlook and then press Windows+R
  • Type Outlook /safe and press ‘Enter’
  • Use Outlook normally and check if the error is fixed

If fixed, run Outlook in normal mode and disable add-ins, one at a time and observe Outlook for the error. This will help you uninstall the faulty add-in and keep non-faulty add-ins.

Method 4: Repair OST file with Inbox Repair Tool

If the error is caused by some problem with OST file such as read/write permission or integrity issue, you can use Microsoft’s inbuilt Outlook data file inbox repair tool called ScanPST.exe.

Earlier, Microsoft used to provide ScanOST.exe to fix OST problems. But ScanOST is discontinued from Outlook 2010.

By repairing the Outlook OST file with ScanPST.exe, you may get rid of the error – “The file xxxx.ost is in use and cannot be accessed”. Here are the steps to use this utility,

  • Close Outlook and launch ScanPST.exe tool. To find ScanPST.exe, browse these locations based on Outlook version installed on your system,
    • exe location for Outlook 2019/Office 365
      • C:\Program Files\Microsoft Office\root\office16\
    • exe location for Outlook 2016
      • 32-bit Outlook: C:\Program Files (x86)\Microsoft Office\Office16\
      • 64-bit Outlook: C:\Program Files\Microsoft Office\Office16\
    • exe location for Outlook 2013
      • 32-bit Outlook: C:\Program Files (x86)\Microsoft Office\Office15\
      • 64-bit Outlook: C:\Program Files\Microsoft Office\Office15\
    • exe location for Outlook 2010
      • 32-bit Outlook: C:\Program Files (x86)\Microsoft office\Office 14\
      • 64-bit Outlook: C:\Program Files\Microsoft office\Office 14\
    • Click ‘Browse’ and navigate to the directory on your system where Outlook OST is stored. The default location is C:\Users\%username%\AppData\Local\Microsoft\Outlook

  • Click ‘Start’. After the scan, if the tool finds errors, click ‘Repair

After repair, start Outlook and check if the error is fixed. 

NOTE: ScanPST.exe does not work for orphaned and encrypted OST files and file size above 2 GB.

Method 5: Recreate OST File

If the OST file is still inaccessible and the error isn’t resolved, you can try to recreate the OST file. This can help in rectifying minor corruption issues within OST file.

To do so,

  • Close Outlook and go to the OST file location
  • Then rename the OST file extension with .bak extension

For instance, if the OST file name is abc@xyz.com.ost, rename it to abc@xyz.com.ost.bak

  • Then open Outlook. This will re-create the OST file and sync the mailbox items

In case this also doesn’t work, then there’s no other option but to convert the OST file data to PST. PST is also an Outlook data file which can be imported in any Outlook profile. The process is explained in the next method.

Method 5: Convert OST to PST

To convert OST to PST, there are two ways,

Export OST to PST via Outlook Import Export Wizard

If you can access the Outlook, you may try these steps to export OST to PST file via Outlook Import/Export wizard.

  • Launch Outlook and go to File> Open> Import
  • This will open the ‘Import and Export Wizard’
  • Select ‘Export to a file’ and click on the ‘Next’ button

  • From the next screen select ‘Outlook data file (.pst)’ and click on ‘Next’

  • Click on ‘Browse’ to pick a destination to save converted file and click on ‘Finish’

Note: Since the OST file is inaccessible, the Outlook Import/Export wizard may not work. In such case, use the OST to PST converter tool.

Convert OST to PST by using Stellar Converter for OST

Stellar Converter for OST makes the process of converting the inaccessible OST to PST quite easy. With its intuitive graphical user interface and useful features, an Outlook user can easily export the mailbox in OST file to PST, even if the file is orphaned or encrypted. It converts OST to PST with 100% data integrity and precision.

This tool can help you prevent “The file username.ost is in use and cannot be accessed” error and other errors associated with OST files. You can download the demo version to scan and preview all your mailbox component in OST file for free before saving it to .pst file.

Further, you may choose to export OST mailbox items to other file formats. There’s a Technician version also that can export your OST mailbox to Live Exchange server, Office 365 account or Outlook profile directly.

Wrapping Things Up

The guide helps you resolve “The file username.ost is in use and cannot be accessed” and making OST file accessible through various troubleshooting methods. However, the best and reliable way to fix the error and avoid further damage to the OST file, use OST to PST converter tool such as Stellar Converter for OST.


But you may try the troubleshooting methods at your discretion. If they fail, don’t hesitate to use this trustworthy product. Read: What MVPs say about Stellar Converter for OST.

MVP Review on Stellar Converter for ost

Read: What MVP’s says about Stellar Converter for ost.

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