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In classic Outlook for Windows, the default OST folder is %LOCALAPPDATA%MicrosoftOutlook—usually C:Users<username>AppDataLocalMicrosoftOutlook. Press Windows key + R, enter the first path, and press Enter. These instructions are for classic Outlook; new Outlook for Windows and Outlook for Mac use different cache workflows.
Default OST file location in Windows
For supported classic Outlook versions, including Outlook for Microsoft 365, Outlook 2024, 2021, 2019, and 2016, the default folder is:
C:Users<username>AppDataLocalMicrosoftOutlookThe quickest way to open it is to paste this environment-variable path into the Windows Run box or File Explorer address bar:
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The file itself may not be named outlook.ost. Its name can reflect the account, mailbox, or profile, and a computer with multiple accounts or Outlook profiles may have several OST files.
Open the folder with Run
- Press Windows key + R.
- Enter
%LOCALAPPDATA%MicrosoftOutlook. - Press Enter.
You can also open File Explorer, paste the same path into its address bar, and press Enter.
Show hidden files and file extensions
If you browse manually or cannot tell which file is an OST, enable hidden items and extensions. In Windows 11 File Explorer, use View > Show > Hidden items and View > Show > File name extensions. In some Windows builds, use Control Panel > File Explorer Options > View, select Show hidden files, folders, and drives, clear Hide extensions for known file types, then select OK. Labels may vary slightly by build. Microsoft also notes that hidden folders and extensions can make Outlook data files harder to identify (find and transfer Outlook data files).
Find and confirm the OST for an account
To identify which file belongs to an account, check the data-file listing in classic Outlook rather than guessing from filenames:
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- Open Outlook and select File > Account Settings > Account Settings.
- Open the Data Files tab and select the relevant entry.
- Use the available settings or file-location control to inspect its location.
Depending on the Outlook build and account type, an Exchange or Microsoft 365 account may expose the offline-file path through Change > More Settings > Advanced > Offline Folder File Settings. Button names and availability vary with the build, account configuration, and organization policy. If you have multiple profiles, verify the profile currently open: different profiles can use different data files.
What an OST file is—and how it differs from a PST
OST means Offline Outlook Data File. In classic Outlook, it is generally a local, synchronized cache of mailbox data for supported Exchange, Microsoft 365, or Outlook.com configurations, and some IMAP setups. It lets Outlook work with cached items when the connection is slow or unavailable. Account behavior depends on the Outlook version and configuration; POP accounts generally use PST files instead. An OST is not the master mailbox and is not designed as a portable backup.
| File | Main purpose | Portable? | Common context |
|---|---|---|---|
.ost |
Local synchronized offline cache | No, not as a standalone archive | Classic Outlook accounts using an offline cache |
.pst |
Local data store, archive, or export | Yes, with appropriate handling | POP accounts, archives, and supported exports |
A PST is commonly found under C:Users<username>DocumentsOutlook Files; that is not the standard OST folder. If you found a PST there, you have found a different kind of Outlook data file. For Microsoft’s explanation of the distinction and transfer behavior, see its data-file guidance.
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Does new Outlook for Windows use an OST?
The classic Outlook path and steps above do not automatically apply to new Outlook for Windows. Microsoft says Cached Exchange Mode is unavailable in new Outlook; this is the classic Outlook feature associated with the traditional Exchange OST workflow. That does not establish that new Outlook has no local application data—it means you should not look for or manage its data using the classic OST instructions.
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Microsoft’s visual distinction is that new Outlook is labeled Outlook beside the Outlook icon, while classic Outlook shows only the Outlook icon in the top app bar. See Microsoft’s guide to identifying Outlook and finding data files.
Where is the Outlook cache on Mac?
Do not use the Windows OST path on a Mac. Outlook for Mac manages its cache differently; Microsoft documents in-app cache actions rather than the classic Windows OST workflow. For an Exchange folder, Control-click or right-click it and choose Properties > Empty Cache. To reset an account, use Tools > Accounts, select the account, then choose Manage > Reset Account. These actions can remove data that has not synchronized to the server, so confirm synchronization before resetting or emptying anything. See Microsoft’s Outlook for Mac cache instructions.
Can you move, delete, or recreate an OST?
Do not manually move an OST while Outlook is running, and do not copy it to another computer as a migration method. The file is tied to the profile and account that created it. When an account is connected again, Outlook can generally create and synchronize a new cache. Unsynchronized local changes may not come back, so confirm synchronization before replacing, renaming, or resetting an OST.
If you need to change the configured location in classic Outlook, Microsoft’s procedure uses the account’s offline-folder settings, not a simple file move:
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- Exit Outlook.
- Open Control Panel > Mail and select the relevant profile.
- Select Email Accounts, choose the Exchange account, and select Change.
- Open More Settings > Advanced > Offline Folder File Settings, then specify the desired location if the control is available.
- Restart Outlook and allow it to create or synchronize the offline file as needed.
Controls vary by version and policy, and some organizations may prevent changing the location. A newly specified location can result in a new OST, so Outlook may need to download cached mailbox data again. For the supported procedure and limitations, see Microsoft’s OST guidance.
If the OST is missing or Outlook says it is corrupt
- Confirm you are using classic Outlook and that the account is configured to use an offline cache.
- Open
%LOCALAPPDATA%MicrosoftOutlook. You can also search File Explorer for*.ost. - Check the Outlook profile and account, and consider whether the file was redirected, moved, deleted, or created under another Windows user.
- If the mailbox is server-based, first confirm recent mail, calendar, contacts, and other changes have synchronized.
- Close Outlook. If the file is suspected to be corrupt, rename it (for example,
account-old.ost) rather than immediately deleting it. - Reopen Outlook and let it create a fresh cache and synchronize. Check that recent items are present before removing the renamed file.
Microsoft describes disabling and re-enabling Cached Exchange Mode as a way to recreate a server-backed OST. Rebuilding can restore synchronized server data, but it cannot guarantee recovery of changes that existed only in the old local cache. The Inbox Repair Tool, SCANPST.EXE, should not be treated as a universal repair solution for every OST; replacing a server-synchronized cache is generally the relevant first step. See Microsoft’s data-file repair guidance.
Should you back up an OST?
No—do not rely on an OST as your backup. It is normally a synchronized local copy of data held on the mail server and can generally be recreated by reconnecting the account. If you need an independent archive or portable copy, use a supported PST export or your organization’s approved retention and backup system.
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Why you may not find an OST
- You are using new Outlook: its workflow is not classic Outlook’s Cached Exchange Mode and OST path.
- No OST has been created: the account may not use an OST, Cached Exchange Mode may be off, or the account may not yet have initialized its cache.
- The folder or extension is hidden: use the environment-variable path and show hidden items and file extensions.
- You are in the wrong profile or Windows account: check the active Outlook profile; multiple profiles can have separate files.
- The location differs: Windows may be on another drive, a policy may redirect the file, or the file may have been moved.
- You searched the PST folder:
DocumentsOutlook Filesis a common PST location, not the standard OST location.
For an older Windows installation, a legacy location was C:Documents and Settings<username>Local SettingsApplication DataMicrosoftOutlook. This is relevant mainly when checking an old disk or legacy system, not a normal Windows 10 or 11 installation.
If the OST is very large
Microsoft warns that OST files of 25 GB or larger can contribute to more frequent short pauses, particularly while new messages download. Practical options include reducing the amount of mail kept offline, archiving or moving older items according to organizational policy, and removing unneeded large messages or attachments. Do not delete an OST as a size-reduction shortcut; Outlook may simply rebuild it, and unsynchronized changes may be lost.
After deleting enough data, compacting may reclaim unused space. Microsoft says Outlook can compact automatically during idle time when more than 20% of a data file is unused. In classic Outlook, manual compaction is available at File > Account Settings > Account Settings > Data Files > Settings > Compact Now. Controls may vary by account and build. See Microsoft’s guidance on reducing and compacting data files and its Outlook performance troubleshooting guidance.
Frequently Asked Questions
Where is the OST file in Windows 11?
In classic Outlook for Windows, open %LOCALAPPDATA%MicrosoftOutlook. It normally resolves to C:Users<username>AppDataLocalMicrosoftOutlook.
Can I open an OST file without Outlook or copy it to another computer?
Usually not as an independent mailbox archive. An OST is tied to the Outlook profile and account that created it; reconnect the mailbox and let Outlook rebuild its cache instead of treating the file like a portable PST.
Is it safe to delete an OST?
Only after confirming that recent changes have synchronized and Outlook is closed. Outlook can generally recreate a server-backed cache, but unsynchronized local changes may be lost; renaming the file first is a safer precaution.
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