Resolving ‘Outlook OST File cannot be Opened’ Error

Microsoft Outlook is one of the most popular email applications and is preferred by businesses as well as individuals for communication and collaboration. Outlook uses PST and OST file formats for storing the mailbox data, depending on the server. Sometimes, you may encounter issues and errors with your Outlook application or the data file. One such error is “Outlook OST file cannot be opened.” 

This error may occur due to different reasons, such as OST data file corruption, interruption while sharing OST file across the network, synchronization issues between OST file and the server, improper shutdown of the Outlook, incompatible Outlook version, Outlook profile corruption, etc. Whatever be the reason for the Outlook OST file cannot be opened error, let’s know the potential solutions to fix this error.

Methods to Fix the ‘Outlook OST File cannot be Opened’ Error

Below are some effective solutions to fix the Outlook OST file cannot be opened error. 

Note: Before proceeding ahead, take a backup of the OST file.

  • Method 1: Close Outlook Application and Related Processes

If too many processes related to Outlook are running on the system, then it may also lead to the error. To resolve this, you can close all the processes related to Outlook that are running in the background. To do so, follow the steps below:

  • First, click File > Exit to close Outlook.
  • Then, press the Ctrl+Shift+Esc keys to open Task Manager.
  • Navigate to Microsoft Outlook in the processes section.
  • If any Outlook-related processes are running, select them and click End Task.
  • Close the Task Manager.
  • Reopen Microsoft Outlook and check if the error is resolved.
  • Method 2: Turn off Cached Exchange Mode in Outlook

Cached Exchange Mode is a default feature in Microsoft Outlook. It stores a local copy (OST) of your Exchange Server mailbox. It helps access the mailbox data when the internet connection is not available. Disabling Cached Exchange Mode may help in resolving the OST file not opening error. To disable the mode, follow these steps:

  • In Outlook, click the File menu and then Account Settings.
  • Select Account Settings to open the Email Accounts window. 
  • Select the email account and click Change.
  • Click More Settings and then click the Advanced tab.
  • Clear the settings of Clear Exchange Mode, including the shared and public folder favorites. 
Turn off Cached Exchange Mode
  • Close and reopen Outlook to check if the error has been resolved.
  • Method 3: Disable Faulty or Conflicting Add-ins

Faulty or conflicting Outlook add-ins may also cause the OST file cannot be opened error. To check if add-ins are causing the error, you need to open Outlook in safe mode. Here are the steps:

  • Close all the processes related to Outlook.
  • Press Windows+R to open the Run dialog box.
  • Type “outlook.exe/safe” and press Enter.
  • This will open Microsoft Outlook in safe mode.

If Outlook is opened in safe mode without any error, it means some add-ins are causing the issue. In such a case, you can disable the add-ins to resolve the issue. Follow the below steps: 

  • In Outlook, click on the File menu and then click Options.
  • Under the Manage box, click “Com Add-ins” and click Go.
  • Uncheck the boxes next to add-ins that could be incompatible and click OK. 
  • Close and reopen Outlook to check if the issue has been resolved.
  • Method 4: Repair and Reinstall the Outlook 

You may encounter the Outlook OST file opening error if there is an issue with your Outlook program. In such a case, you can try repairing your Outlook program. If it does not work, reinstall the Outlook application. 

Steps to Repair Outlook Application

  • Go to Control Panel.
  • Click Programs and Features. 
  • Right-click on Microsoft Office and select Change. 
  • In the Microsoft Office Setup window, select Repair and click Continue.
  • Complete the repair process by following the instructions given on the screen.

Steps to Reinstall Outlook Application

  • Visit the Microsoft website and download the latest version of Outlook.
  • Run the setup file and complete the installation process by following the on-screen instructions.
  • Method 5 – Create a New Outlook Email Profile

If there are any issues with the Outlook profile, you may encounter issues with the OST file. In such a case, you need to remove the existing Outlook profile and create a new profile. Here’s how:

  • Go to Control Panel.
  • Click on Mail (Microsoft Outlook).
  • In the Mail Setup window, click Show Profiles.
  • Select the associated Outlook profile and click Remove.
  • Again, open the Mail Setup window, click Show Profiles, and then click Add.
  • Enter the name of the new email profile and click OK.
  • Complete the process by following the instructions given on the screen.
  • Launch Microsoft Office to check if the error has been resolved.
  • Method 6 – Disable Outlook Compatibility Mode

The Outlook OST file opening error may also occur due to compatibility issues in Outlook. To resolve this issue, you can disable the compatibility mode. Follow the steps below:

  • Locate the shortcut of Microsoft Outlook on your desktop.
  • Right-click on the shortcut and click Properties.
  • Click on the Compatibility tab and uncheck the box next to “Run this program in compatibility mode for.”
  • Click Apply and then click OK to save the changes.


In Outlook, OST files may not open due to outdated Outlook versions, faulty add-ins, synchronization issues, corrupt or damaged Outlook profile, etc. You can follow the above-mentioned methods to resolve the Outlook OST file not opening error. If the problem persists, then it means the OST file is corrupted or damaged. In this case, you can recreate the OST file. For this, take the backup of the OST file, delete the file from the default location, and then launch Outlook. It will create a new OST file at the same location and synchronize mail items from the server. Then, check the OST file for any missing items. If there are any, then you can use a professional OST to PST converter like Stellar Converter for OST to extract the missing items from the backup OST file and save them in Outlook PST format that you can import easily into your Outlook account. The software can also convert orphaned or inaccessible OST files of any size into PST format.