How to Resolve Error "Outlook.ost is in use and cannot be accessed!"

Exchange account holders who access their mailboxes locally through MS Outlook are pretty well familiar with OST files. An OST file is created automatically when Cached Exchange Mode is turned on.
It stores an exact replica of the Exchange mailbox on the local machine and enables users to continue working even if the server is not available due to technical issues. All modifications made to the OST file are merged with the actual Exchange mailbox when the server comes back online. Thus OST files are quite crucial as far as Exchange mailbox data is concerned. Any damage to them could mean mailbox inaccessibility.

Most Outlook users are accustomed to facing glitches in the application from time to time. Sometimes, the cause behind issues such as Outlook crashes or Outlook working improperly is data corruption at the OST file level. However more often than not, users are unaware of how to identify that OST file is damaged, what is the real cause behind OST file corruption and how to fix it. This article attempts to answer all these questions.

Identifying OST Corruption

The most common indicative sign of OST file corruption is the cropping up of the error "Outlook.ost is in use and cannot be accessed". This error appears on attempting to open Outlook and even if you try to re-launch Outlook several times, the error appears again. Other common error messages that point at OST file corruption are:

• Default Outlook profile OST is in use and cannot be accessed
• Cannot display the folder. Outlook.ost is in use and cannot be accessed
• OST file is in use by another application

As a normal user, the first thought on encountering this error would be to close any application that is using the OST file. However, at times, this may not work and even after quitting Outlook, the error may flash again with the same message. Hence, identifying the real cause behind the error and eliminating it is the only option left.

Reasons behind error "Outlook.ost is in use and cannot be accessed"

• Interrupted or faulty synchronization of OST with Exchange mailbox
• Running of conflicting applications with Outlook
• Unauthorized attempts to access the OST file
• Multiple users trying to access the same OST file
• Unreliable internet connection

How to fix Outlook Error

There are 4 ways in which you can repair OST file and fix this Outlook error.

1. Convert OST to Outlook PST

Converting OST to PST is the most recommended solution to resolve the error. This can be done manually or through automated third-party tool.

A. Manual method

Run Outlook in Safe mode and then follow the below mentioned steps:

• Click on File -> Open -> Import
• In the pop-up window that opens, select 'Import from another program or file'
• In the next pop-up, select ‘Outlook Data File (.pst)
• Click Next
• Click on Browse and then select the OST file that needs to be converted
• Select the appropriate option to replace, allow or ignore import of duplicate items
• Click Next
• Provide desired location to store the converted file
• Click Finish

Note: Manual conversion of OST to PST runs the risk of data loss. Some mailbox data (email attachments etc.) or mailbox data structure might get lost or altered while conversion. It is also a lengthier process that takes time. Moreover, it is not available in Outlook versions later than 2007. 

Hence to overcome these limitations, use automated third-party OST to PST Conversion tool mentioned below.

B. Automated third-party tool

Stellar OST to PST Converter is the tool we recommend to achieve this task. This reliable application quickly resolves all types of Outlook errors and converts all objects within the OST file to PST format. The converted PST file can then be directly imported into Outlook to access mailbox data. In addition to PST format, this software allows you to convert OST data to MSG, EML, RTF, HTML, and PDF formats as well.

2. Use Inbox Repair Utility ScanOST.exe to repair OST

Microsoft offers an inbuilt utility to repair damaged Outlook OST. This utility can perform Outlook OST recovery by repairing OST file. This utility is also known as the "OST Integrity Check" tool. To use this tool:

• Quit Outlook through Task Manager if it is running
• Locate ScanOST.exe on your computer and launch it. It's default location is <MicrosoftInstallationFolder>\<OfficeVersion>\ScanOST. For example, C:\Program Files\Microsoft Office\Office12\SCANOST.
• Type Outlook Profile Name when the tool prompts for it
• Choose Connect if the tool prompts
• Select desired options ("Scan all folders" etc.)
• Select the “Repair Errors” checkbox to make the tool automatically check and remove errors.  If you do not select this checkbox, the tool will detect issues but won’t correct them.
• Click on "Begin Scan"

This utility is available in Outlook 2007 and earlier versions only. Moreover, it is designed to tackle minor issues with the OST files. For major corruption issues in the OST, the next option is most recommended.

3. Disable Cached Exchange Mode

As already mentioned, OST file gets automatically created when Cached Exchange Mode is turned on. So turning it off might solve the issue. To do this:

• Quit Outlook through Task Manager if it is running.
• Open Control Panel -> Mail -> Email Accounts
• Under the Email tab highlight the email address that’s giving you the warning (Email.ost)
• Select Change
• Uncheck "Use Cached Exchange Mode"
• Select Next -> Finish
• Now try to re-launch Outlook.

Final Words

We are sure if none of the free methods work, the premium automated software will definitely resolve this error. And in our opinion paying a little to get things back on track shouldn't be a problem. Here's hoping you are able to fix all OST errors and continue your work.