"The file username.ost is in use and cannot be accessed"

Microsoft Outlook is a widely used email client application used by individuals and large organizations to manage their mailbox item. Facing any issue while working with Microsoft Outlook can be frustrating and complicated. Sometime when users try to open an OST file in MS Outlook encountered an error stated as:


"Outlook Data File – The file C:\Users\username\AppData\Local\Microsoft\username\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 your computer."

Fix OST error

It restricts the user to open the OST file and continue their work. This OST file error usually occurs when the user is trying to access the same OST file that is being used by another application. It also happens when users opt for the "Save and Send" option. The error is usually solved by restarting the system as suggested in the error itself. Also closing all other related programs may fix the error.

Reasons for the occurrence of the "OST is in use and cannot be accessed" issue

This error mainly occurs when multiple applications or users attempt to access the same OST file. Some other processes like UCMAPI.EXE, Office Communicator 2005 or 2007, LYNC, etc. are running in the background using the same OST file. Apart from this, there can be several other reasons also that can cause this error, some of them are listed below:

  1. Issue While Synchronization between the OST file and Exchange Server.

  2. Due to Improper internet connection.

  3. If an unauthorized user tries to access to OST file.

Troubleshooting methods to solve “OST is in use and cannot be accessed” Issue

Solution-1: End all Outlook and other Outlook related tasks or processes.

As the error message states that OST is in use by other Outlook related processes, there are more chances that the error can be solved by ending all MS Outlook related processes and tasks then restart them.

Follow the step given below and terminate all other tasks:

  1. First of all, Close MS Outlook. 

  2. After that, press Ctrl+Alt+Del keys together and select the “Task Manager”.

  3. In the opened task bar, choose “Processes” tab.

  4. Now, in the list search for processes like Outlook, lync, ucmapi communicator, etc. or which you think can interrupt the MS Outlook application, then hit on End task button one by one and end all process.

  5. Now, restart the system and open MS Outlook.



Solution- 2: Disable Exchange Cached Mode option

Another way to fix this error is to disable off Exchange cached mode. This mode allows user to work in offline mode but disabling it temporarily can solve the error, for that perform the following steps:

  1. First of all, close your Microsoft Outlook application and open Control Panel.

  2. Now go to “Mail” option and then a new wizard will appear, navigate the “Email Accounts” option.

  3. After that select “Email” option.

  4. Here, select your email account that is connected and click on the “Change” button.

  5. Then, simply uncheck the box of  Use Cached Exchange Mode option to disable it.

  6. Click on the Next and then on  Finish button to complete the process.



Solution- 4: Convert OST to PST

This two solutions might resolve the error. However, if none of the above trick are unable to solve the error then the only possible solution to fix it is converting faulty OST file to PST format. There exist two ways to export OST files to Outlook data file PST format by ScanOST Option or third party tool. The ScanOST wizard can only fix minor issue related to OST file. However an OST to PST converter software is compatible to repair OST files with both minor and major level corruption.


From all the above different solutions using a Professional Tool is most reliable option to fix the error “cannot display the folder the file outlook ost is in use and cannot be accessed”.  In case above mentioned methods fails to repair the issue or there is a risk of data loss; you can use Magus OST to PST conversion tool. It will help to fix the OST file error by converting into .PST format which can be further opened or imported into Microsoft Outlook.