If Outlook suddenly refuses to start or stops sending and receiving mail with the message “The file .ost cannot be opened,” your offline data file has become unreadable. This is one of the most common Outlook data file errors, and in most cases you can resolve it in a few minutes without losing a single email. This guide explains what the error means, why it happens, and how to fix it safely, including what to do when your OST file is orphaned or badly corrupt.
Table Of Contents
- 1 Quick answer
- 2 What does “The file .ost cannot be opened” mean?
- 3 Why does the OST file cannot be opened error occur?
- 4 Before you start: protect your data
- 5 How to fix the “file .ost cannot be opened” error
- 5.1 Method 1: Fully close Outlook and reopen it
- 5.2 Method 2: Start Outlook in Safe Mode and turn off add ins
- 5.3 Method 3: Remove the read only attribute and check permissions
- 5.4 Method 4: Rename the OST file so Outlook rebuilds it
- 5.5 Method 5: Create a new Outlook profile
- 5.6 Method 6: Repair Office and update Outlook
- 5.7 Method 7: Stop your antivirus from locking the OST
- 6 When manual fixes do not work: recover the OST data
- 7 How to prevent the OST file cannot be opened error
- 8 Conclusion
- 9 Frequently asked questions
Quick answer
The “file .ost cannot be opened” error appears when Outlook cannot read your offline OST file. The usual reasons are a corrupt OST file, an orphaned OST file, a file that is locked by another process, or a damaged Outlook profile. If your mailbox still exists on the Exchange or Microsoft 365 server, the fastest fix is to close Outlook, rename the OST file, and let Outlook rebuild a fresh copy. If the OST file is orphaned or corrupt and holds data that is no longer on the server, recover it to PST first so nothing is lost, then apply the fixes below.
What does “The file .ost cannot be opened” mean?
An OST file (Offline Storage Table) is a local copy of your mailbox that Outlook creates when your account runs in Cached Exchange Mode with Exchange, Microsoft 365, or Outlook.com. It lets you read and write mail while offline, then syncs the changes back to the server when you reconnect.
The error means Outlook located the OST file but could not open or read it. You may see it in one of these forms:
- “The file C:\Users\YourName…\outlook.ost cannot be opened.”
- “The file .ost is in use and cannot be accessed.”
- “Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened.”
If you are unsure what the difference between the two Outlook data files is, our guide on the difference between OST and PST files explains it clearly.
Why does the OST file cannot be opened error occur?
The error has several possible causes. Identifying yours points you to the right fix.
| Cause | What is happening | Typical sign |
|---|---|---|
| OST file corruption | Internal damage stops Outlook reading the file | Error on startup or during send and receive |
| Orphaned OST file | The mailbox or profile no longer exists on the server | Error after an account change, migration, or employee exit |
| File locked or in use | Another process is holding the OST open | The “in use and cannot be accessed” variant |
| Damaged Outlook profile | The profile points to a bad OST path or settings | Error even when the OST file itself is healthy |
| Read only or permission issue | Outlook cannot write to the OST file | Error after moving the file or a Windows update |
| Antivirus or firewall lock | Security software scans and locks the OST | Intermittent error during send and receive |
| Outdated Outlook after a server upgrade | Version mismatch with the upgraded Exchange | Error following a server migration |
| OST stored on a network drive | An unsupported location for OST files | Frequent open or sync failures |
Before you start: protect your data
Important: If your OST file is orphaned, meaning the mailbox or account no longer exists on the server, do not delete or rename it yet. That file may be the only remaining copy of your emails. Recover the data to PST first using the recovery method further down this page, and only then apply the other fixes.
If your mailbox is still active on the server, it is safe to proceed, because Outlook can always download a fresh copy.
How to fix the “file .ost cannot be opened” error
Work through these methods in order. The early ones are quick and safe, and most users are back up and running within the first three.
Method 1: Fully close Outlook and reopen it
A lingering background process is the most frequent reason the OST file appears locked.
- Close Outlook.
- Open Task Manager by pressing Ctrl, Shift, and Esc together.
- On the Processes tab, look for any entry named Outlook or outlook.exe.
- Select each one and click End task.
- Reopen Outlook and check whether the error is gone.
Method 2: Start Outlook in Safe Mode and turn off add ins
A faulty extension can block Outlook from opening the OST file.
- Press the Windows key and R together to open the Run box.
- Type outlook.exe /safe and press Enter.
- If Outlook opens without the error, an add in is the cause.
- Go to File, then Options, then the section for add ins.
- Next to “COM Add ins,” click Go, clear every checkbox, and click OK.
- Restart Outlook normally and enable the extensions one at a time to find the culprit.
Method 3: Remove the read only attribute and check permissions
If Outlook cannot write to the file, it cannot open it either.
- First locate the file. Our guide on how to find your OST file location in Outlook shows the exact path for every version.
- Right click the OST file and choose Properties.
- On the General tab, clear the Read only checkbox and click Apply.
- Open the Security tab and confirm your Windows account has Full control.
Method 4: Rename the OST file so Outlook rebuilds it
Use this only when your mailbox is still active on the server. Outlook will create a new OST file and download everything again.
- Close Outlook completely (repeat Method 1 if needed).
- Browse to the OST folder you found in Method 3.
- Rename outlook.ost to outlook.old.
- Reopen Outlook. It builds a fresh OST file and syncs your mailbox from the server.
Method 5: Create a new Outlook profile
A corrupt profile can trigger the error even when the OST file is fine.
- Close Outlook and open Control Panel.
- Click Mail, then Show Profiles.
- Click Add, name the new profile, and add your email account.
- Set the new profile as the default and open Outlook.
Method 6: Repair Office and update Outlook
A damaged installation or a version mismatch after a server upgrade can cause the error.
- Open Control Panel, then Programs and Features.
- Select Microsoft Office and click Change.
- Choose Online Repair and follow the prompts.
- Afterward, open Outlook, go to File, then Office Account, then Update Options, and install any updates.
Method 7: Stop your antivirus from locking the OST
Real time scanning sometimes holds the OST file open at the moment Outlook needs it.
- Open your antivirus settings and find the exclusions or exceptions list.
- Add your OST file, or the folder that contains it, as an exclusion.
- Restart Outlook and test send and receive.
When manual fixes do not work: recover the OST data
Two situations resist the steps above. The first is an orphaned OST file, where the mailbox has been deleted or the account deactivated, so there is no server copy to rebuild from. The second is severe corruption, where renaming and repairing do not help and Outlook still cannot open the file.
In both cases the safe route is to extract your mailbox data out of the inaccessible OST file and into a healthy PST file that opens in any Outlook version. A professional OST to PST Converter reads the offline file directly, without needing Outlook or a live Exchange connection, and exports everything to PST. For files that are deeply damaged, an OST Recovery tool repairs the structure and restores emails, contacts, calendars, and attachments before saving them.
The workflow is straightforward:
- Install and open the tool.
- Add the inaccessible or orphaned OST file.
- Preview the recovered mailbox items to confirm they are intact.
- Export to PST, or send the data straight to Office 365, Exchange, or another email client.
- Open the resulting PST in Outlook, or convert further as needed.
If you would like the full walkthrough, see our step by step guide on how to convert OST to PST, which covers both the manual export method and the automated tool.
How to prevent the OST file cannot be opened error
A few habits keep this error from returning.
- Always close Outlook properly before shutting down or restarting Windows.
- Keep Outlook and Office current so they stay compatible with your server.
- Store the OST file on a local drive, never on a network or shared location.
- Archive old mail and empty the Deleted Items folder so the OST file does not grow oversized.
- Convert important OST data to PST from time to time as a backup, so a server outage or corruption never locks you out of your emails.
For a wider reference, our roundup of common Outlook errors and their solutions covers the other messages you may run into.
Conclusion
The “file .ost cannot be opened” error looks alarming, but it is almost always fixable. Start by closing every Outlook process, rule out a bad add in with Safe Mode, and clear any read only or permission block. If your mailbox is still on the server, renaming the OST file to force a rebuild is the cleanest fix. When the file is orphaned or corrupt, recover the data to PST first so nothing is lost, then rebuild. Follow the prevention tips and this error is unlikely to trouble you again.
Frequently asked questions
It means Outlook found your offline OST file but could not read or open it. The common causes are corruption, an orphaned file with no matching mailbox on the server, a file locked by another process, or a damaged Outlook profile.
Yes, in most cases. If your mailbox is still active on the server, renaming the OST file lets Outlook download a fresh, complete copy. You only risk data loss when the OST file is orphaned, which is why you should recover it to PST before deleting or renaming anything.
ScanPST.exe, the Inbox Repair Tool, is designed mainly for PST files. For OST problems the standard fix is to rebuild the OST file from the server, and for orphaned or badly corrupt OST files a dedicated recovery or conversion tool is the reliable option. You can still learn how to download and use ScanPST.exe for your PST files.
An orphaned OST file is one whose mailbox or account no longer exists on the server, so Outlook has nothing to sync it with. Outlook cannot open it directly, but an OST to PST Converter can read the file offline and export the data to a usable PST.
By default it sits under your user profile in the Outlook data folder. The exact path varies by Outlook version, and our guide on how to find your OST file location lists each one.
It is safe only when your mailbox is still on the server, because Outlook simply rebuilds it. If the OST file is orphaned, deleting it permanently destroys any emails that were never synced back to the server, so recover the data first.
Use an OST to PST Converter that reads the offline file without Outlook, preview the items, and export them to PST. The detailed steps are in our convert OST to PST guide.
No. Creating a new profile does not touch your server mailbox. Outlook downloads your mail into the new profile, and any locally stored PST files remain untouched.
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