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Fix 'OST File Is in Use and Cannot Be Accessed' in Outlook

Start by fully closing Outlook and any process still holding the file, then restart if a stale lock persists. If that doesn't help, try Safe Mode to rule out an add-in. If several PCs show the same error on one shared mailbox, the cause is more likely server/Autodiscover configuration than a local file problem. If the live mailbox is healthy and the old OST is simply misbehaving, rebuild the profile/OST after preserving any unique local data. Only use EdbMails recovery as a fallback, once Outlook and other programs have released the file.

When Outlook shows that the OST file is in use and cannot be accessed, it typically tells you to close any application using the file and try again. This message points to a file lock or a background process still holding the OST open—not automatically to corruption. Treating it as a corruption problem first can send you down the wrong troubleshooting path.

This guide works through the causes in the order you should actually check them: file locks and background processes first, then profile and server/shared-mailbox configuration, and only then corruption. Where the OST itself holds data that Outlook can no longer reach any other way, we'll also cover EdbMails OST to PST Converter as a fallback recovery option.

Outlook data file cannot be opened OST

Diagnose which branch applies to you

Condition	Correct route	Why
Outlook or another process is still using the OST	Close Outlook and the confirmed locking process, then restart Windows if the lock remains.	The error usually indicates that the file is still open or a stale lock has not been released.
Outlook works normally in Safe Mode	Disable Outlook add-ins one at a time and retest.	Safe Mode disables add-ins and customizations, helping isolate an add-in conflict.
The same shared mailbox fails on several PCs but works in Outlook on the web	Check mailbox permissions, Automapping and Autodiscover before changing local OST files.	A multi-device failure points more strongly to mailbox or service configuration than to one local cache.
A new Outlook profile works	Use the new profile or rebuild the affected Outlook profile and OST after preserving any local-only data.	The live mailbox is available, so Outlook can create a fresh local cache from the server.
The live Exchange or Microsoft 365 mailbox is healthy but the old OST still fails	Rebuild the OST after confirming required mailbox data is available on the server.	The server mailbox remains the authoritative source and can repopulate a new OST.
The original mailbox or profile cannot be restored and the OST contains required data	Use EdbMails to scan recoverable OST data after the file is no longer locked.	Recovery is appropriate when the OST is the remaining source of data rather than a replaceable cache.

Find what is holding the OST file open

Before changing the Outlook profile or rebuilding the OST, identify whether Outlook or another authorized process still has the file open. This separates a temporary file lock from a profile, mailbox, or data-file problem.

Condition	Correct route	Why
Outlook remains active after the window is closed	Close Outlook normally, check Task Manager, and restart Windows if the process does not release the file.	An Outlook process can continue holding the OST even after the visible window closes.
A backup, antivirus scan, or another utility is accessing the OST	Allow the authorized operation to finish or close the confirmed process before reopening Outlook.	Another application can temporarily prevent Outlook from obtaining access to the file.
Another Windows session or Outlook profile is using the file	Close the active session or profile that legitimately owns the file.	The same OST should not be accessed concurrently by multiple active sessions.
The lock is cleared but Outlook still cannot use the OST	Continue with Safe Mode, mailbox checks, profile testing, and the later recovery branches below.	A persistent failure after the lock is removed points to a different Outlook, profile, mailbox, or file issue.

How to fix 'OST file is in use and cannot be accessed'
Work through the methods below in order. Most cases resolve at Method 1 or 2; the later branches are for shared-mailbox setups, profile rebuilds, and confirmed corruption.
- Method 1: Close Outlook and any locking process, then restart if needed
  Close Outlook and any other process that is demonstrably holding the OST file open, then try reopening Outlook. If the file stays locked, a clean restart clears a stale lock in most cases.
  - Step 1: Close Outlook on your computer
    Click 'File' > 'Exit' or click the 'X' to close Outlook.
    Open Task Manager by pressing Ctrl + Shift + Esc.
    If Outlook is still listed under Processes, select it and click 'End Task'.
  - Step 2: Close any other process still holding the file
    In Task Manager, check for any other process that is demonstrably accessing the OST file—for example, an antivirus scan in progress, a backup tool, or a mail-enabled add-in process running outside Outlook itself. Only end tasks you can confirm are related; don't end unrelated processes indiscriminately.
    Close Task Manager once the relevant process(es) are ended.
  - Step 3: Reopen Outlook
    Open Microsoft Outlook and check whether it synchronizes normally and the error is gone.
  - Step 4: Restart Windows if the lock persists
    If the file still shows as in use after Step 1–3, restart Windows. A full restart clears a stale file lock or a process that Task Manager couldn't fully release.
- Method 2: Start Outlook in Safe Mode to test for an add-in conflict
  Safe Mode starts Outlook with add-ins and customizations disabled. It's a way to test whether an add-in is holding the file or causing the conflict—it is not a malware removal step, and starting Outlook in Safe Mode does not scan for or remove viruses.
  - Step 1: Start Outlook in Safe Mode
    Press Windows key + R to open the Run dialog.
    Type 'outlook.exe /safe' (without the quotes) and press Enter.
    Outlook starts with only basic functionality loaded and add-ins disabled.
  - Step 2: Check whether the error is gone
    See if Outlook opens the OST and synchronizes normally in Safe Mode.
    If it does, an add-in is the likely cause—continue to Step 3 to isolate which one.
  - Step 3: Disable add-ins one at a time from Outlook options
    In Outlook, click 'File' > 'Options' > 'Add-ins'.
    In the Manage box, select 'COM Add-ins' and click 'Go'.
    Uncheck the add-ins you suspect, click 'OK', then close and reopen Outlook.
    Re-enable add-ins one at a time, restarting Outlook after each, until you find which one reproduces the error.
- Shared mailbox branch: the same error appears for several people on one shared mailbox
  If a shared or public-folder mailbox shows this error on multiple desktops at once, the cause is more likely server-side than a single machine's local file. Microsoft documents this pattern for shared/public-folder mailboxes where Autodiscover or mailbox configuration, rather than local OST corruption, is the root cause.
  - Check whether it works in Outlook on the web (OWA)
    If the shared mailbox opens fine in OWA but fails with this error in Outlook on multiple desktops, investigate mailbox permissions and Autodiscover configuration for that mailbox before repeatedly deleting and recreating OST files on each machine.
    Confirm the affected users have the correct Full Access / Automapping permissions on the shared mailbox.
    Verify Autodiscover is resolving correctly for the shared mailbox in your environment.
  See Microsoft's guidance on Outlook users unable to access public folders / shared mailboxes for the server-side diagnostic steps. If you specifically need to move a shared mailbox's OST data elsewhere, EdbMails can migrate OST data to a live Exchange or Microsoft 365 mailbox, including shared mailboxes.
- Before rebuilding: preserve data that may exist only in the old OST
  Before deleting, renaming, or recreating the OST, confirm that the Exchange or Microsoft 365 mailbox is reachable and contains the data you need. If the old OST may contain local-only or unsynchronized items, preserve the file first. If Outlook can still open the data, export the required items before rebuilding. If the file is orphaned or remains unreadable after the lock is cleared, work from a preserved copy and evaluate its recoverable mailbox data with EdbMails.
- Profile / OST rebuild: the live mailbox is healthy, but the old OST still errors
  Before rebuilding anything, confirm the mailbox itself is healthy—it opens fine in OWA or from another device. If it is, and the old OST on this machine is simply misbehaving, you can safely rebuild it. If the OST has local-only data that hasn't synced to the server, preserve that first.
  - Step 1: Preserve any local-only data first
    If you're not sure everything in the OST has synced to the server, export the items you need to a PST before deleting or recreating the OST. This only works while the OST is still in a healthy, openable state.
  - Step 2: Rebuild the OST or create a new profile
    Go to Control Panel > Mail, click 'Show Profiles', and either remove the existing profile (Outlook recreates the OST from the server) or add a new profile alongside it.
    See how to add a new Outlook profile, or the full Rebuild an OST File in Outlook guide.
    Removing a profile deletes its OST and rebuilds it from the server on next sync—any local-only data not preserved in Step 1 will be lost.
  - Step 3: Repair the email account instead, if only the OST path or config changed
    In Control Panel > Mail > Show Profiles > Properties > Email Accounts, select the account and click 'Repair'. This resets account configuration, including the OST file location, without a full profile rebuild.
- Corruption check: only if there's actual evidence of data-file damage
  Use the Inbox Repair Tool (ScanPST) only when you have real evidence of data-file corruption, such as Outlook reporting errors on specific items or the file failing integrity checks—not merely because the file was locked or in use. A locked file is a Method 1 problem, not a ScanPST problem.
  - If ScanPST can't fully repair the file
    See our dedicated guide to repair a corrupted OST file for the full walkthrough, including when to move to EdbMails recovery instead.
- Other diagnostics worth trying
  These aren't specific to the "file is in use" error, but can help in certain environments:
  - Disabling Cached Exchange Mode changes how Outlook stores data locally and can mask cache-related problems rather than fix them. Treat it as an advanced diagnostic for specific Exchange scenarios, not a default fix.
  - Repairing or updating Outlook from Control Panel > Programs and Features, or installing the latest Outlook build, can resolve errors tied to a corrupted installation rather than the OST itself.
  - Disabling compatibility mode on the Outlook shortcut (right-click > Properties > Compatibility tab) is worth checking if Outlook was recently set to run in compatibility mode after a Windows upgrade.
  - Security software can occasionally interfere with file or process access. Investigate antivirus/firewall settings only if you have specific evidence pointing there, such as the lock reproducing while a scan is active—it is not a common cause on its own.
- EdbMails recovery: fallback when the mailbox or profile can't be restored
  EdbMails OST to PST Converter is a fallback, not a first step. It can recover readable content from an OST when the original mailbox or profile can't be restored, or when you need to extract unique local data another way. It does not unlock a file that's currently held open by another process—Outlook and any other program must release the file first, so run through Methods 1–5 above before turning to EdbMails, or work from a copy of the file if it's still in use.
  EdbMails can scan, recover, and preview an OST file without Outlook or the original Exchange profile. Outlook with a configured profile is only needed if you plan to open the resulting PST in Outlook afterward; it is not required for a direct migration to Microsoft 365 or a live Exchange server.
  1. Make sure Outlook and any other process have released the OST file, or work from a copied/preserved version of it.
  2. Download and install EdbMails, then log in or start the free trial.
  3. Select OST Recovery and Migration > OST to PST and proceed with a job name.
  4. Browse to the OST file, choose a destination folder for the PST, and let EdbMails scan and recover the data it can read.
  5. Preview the recovered mailboxes/folders and select what you need.
  6. Set Action to 'Export' and click 'Start Export' to save the data as a PST file, or choose a direct migration target instead of PST.
  For the full walkthrough with screenshots, see our convert OST to PST guide.

Frequently Asked Questions

What process is using my OST file?
Usually Outlook itself, or a mail-enabled add-in/process still running in the background. Check Task Manager for Outlook and any other process demonstrably tied to it before assuming a specific named process is responsible.
Should I end Outlook.exe in Task Manager?
Yes, if Outlook is still listed as running after you've closed it normally. Avoid ending unrelated processes you can't confirm are holding the file.
Can add-ins cause an OST file lock?
Yes, some add-ins can interfere with how Outlook accesses the file. Starting Outlook in Safe Mode disables add-ins so you can test whether one is responsible.
Why does a shared mailbox show this error on several PCs?
This pattern usually points to server-side configuration—mailbox permissions or Autodiscover—rather than local file corruption on each machine. Check whether the mailbox works in Outlook on the web before troubleshooting each desktop individually.
Should I delete the OST?
Only after confirming the live mailbox is healthy and preserving any local-only data first. Deleting the OST removes it permanently; Outlook rebuilds it from the server, but anything not yet synced is lost.
Does EdbMails unlock a file that is actively in use?
No. EdbMails reads an OST file once it's no longer held open by Outlook or another process. Close Outlook and any other program using the file, or work from a copy, before scanning it with EdbMails.
When should I recover the OST instead of rebuilding it?
Recover it with EdbMails when the original mailbox or profile can't be restored, or when the OST holds local-only data the server no longer has. Rebuild it normally when the live mailbox is healthy and you just need a fresh local cache.