What an Outlook data file is and why you might need to open one

An Outlook data file is a single file that holds your emails, calendar events, contacts, and tasks. Microsoft calls these files PST files (Personal Storage Table) or OST files (Offline Storage Table), depending on the type of account and how Outlook is set up. A PST file is yours to keep and move around. An OST file is a temporary copy that Outlook creates automatically when you use a Microsoft 365 account or Exchange account.

You might need to open a data file if you're switching computers, restoring from a backup, or trying to access emails from an old account that's no longer connected to Outlook. The process differs slightly depending on which version of Outlook you use and what operating system you're on.

Key Takeaways

  • PST files are personal backup files you can move between computers; OST files are temporary copies that Outlook creates automatically and should not be moved.
  • On Windows, use File > Open & Export > Import/Export or drag the PST file directly into Outlook to open it.
  • On Mac, use File > Import to open a PST file; Mac Outlook does not support OST files directly.
  • Before opening a PST file, make sure Outlook is closed completely, or the file may lock and cause errors.
  • If Outlook does not recognize the file, the file may be corrupted, and you may need to use the Inbox Repair Tool (Windows) or contact Microsoft support.

Opening a PST file on Windows

The most straightforward way to open a PST file on Windows is to drag it into Outlook. Close Outlook completely first. Then locate your PST file (usually stored in Documents or Downloads), and drag it into the Outlook window once Outlook is open. Outlook will add the file's contents to your folder list on the left side of the screen.

If dragging does not work, use the Import/Export tool. Open Outlook, go to File > Open & Export > Import/Export. Choose Import from another file or program, select Outlook Data File (.pst), and browse to your file. Outlook will walk you through the rest. This method gives you more control over where the imported emails land in your folder structure.

A third option is to use File > Open & Export > Open Outlook Data File. This opens the PST file as a separate set of folders within Outlook, rather than merging it with your main account. This is useful if you want to keep the old emails visibly separate from your current mail.

Opening a PST file on Mac

Mac Outlook handles PST files through the import process only. Open Outlook, go to File > Import, and select Outlook for Windows Data File. Browse to your PST file and click Import. Outlook will ask where you want the emails to go — you can create a new folder or add them to an existing one.

Mac Outlook does not support opening OST files directly. If you have an OST file and need to access its contents, you will need to either use Windows Outlook or convert the OST to a PST first. The conversion requires Windows Outlook or a third-party tool.

What to do if Outlook will not open the file

The most common reason Outlook refuses to open a data file is that Outlook is still running in the background. Close Outlook completely — not just the window, but the entire process. On Windows, check the taskbar and system tray. On Mac, use Command+Q to quit Outlook fully, not just close the window.

If Outlook still will not open the file, the file may be corrupted. On Windows, use the Inbox Repair Tool (scanpst.exe), which comes with Outlook. Search your computer for "scanpst" or look in your Outlook installation folder. Run the tool, select your PST file, and let it scan and repair. This can take several minutes for large files.

On Mac, there is no built-in repair tool. If a PST file is corrupted, you may need to contact Microsoft support or use a third-party recovery service. Before paying for recovery, try opening the file on a Windows computer if you have access to one, since the Windows repair tool is more robust.

The difference between PST and OST files

A PST file is a backup or archive that you own. You can copy it, move it, email it, or store it on an external drive. If you delete it, the emails are gone unless you have another copy. PST files are what you create when you export emails from Outlook or what older versions of Outlook used as the default storage.

An OST file is a temporary copy that Outlook creates automatically when you connect a Microsoft 365 or Exchange account. Outlook keeps it in sync with the server. If you delete an OST file, Outlook will recreate it the next time you open the process. You should never try to move or open an OST file directly — doing so can cause Outlook to lose sync with the server and create duplicate emails or missing mail.

If you see an OST file and want to back up your emails, export them to a PST file instead. In Outlook, go to File > Save As (or Export on Mac), select the folder you want to save, and choose Outlook Data File as the format.

Where Outlook stores data files by default

On Windows, Outlook stores PST and OST files in C:\Users\[Your Username]\AppData\Local\Microsoft\Outlook. The AppData folder is hidden by default. To see it, open File Explorer, go to View, and check the box for Hidden items.

On Mac, Outlook stores data files in Users/[Your Username]/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile/Data. The Library folder is also hidden. To reach it, open Finder, press Command+Shift+G, and paste the path above.

If you have moved a PST file to a different location and want Outlook to find it, you can either drag it back to the default folder or use the Open Outlook Data File method described earlier, which lets you browse to the file wherever it is.

Frequently Asked Questions

Can I open a PST file without installing Outlook?

No. You need Outlook installed to open a PST file directly. However, you can view PST files using free online converters or third-party tools that read PST format, though these are less reliable than Outlook itself and may not display all content correctly.

What if the PST file is very large and Outlook is slow?

Large PST files (over 2 GB) can slow Outlook down. Consider splitting the file into smaller ones by exporting specific date ranges or folders to separate PST files. You can then open them one at a time or keep them in separate Outlook profiles.

Can I open a PST file from an external drive or USB stick?

Yes, but keep the file on the external drive only while you are actively using it. Outlook performs better when the PST file is on your computer's internal drive. If you want to keep it on external storage, copy it to your computer first, open it, and then copy it back when you are done.

Will opening a PST file delete the original file?

No. Opening a PST file in Outlook does not delete or move the original file. The file stays where it is. If you want to remove it after importing, you must delete it manually.

What does it mean if Outlook says the file is "in use"?

This means Outlook or another program is accessing the file. Close Outlook completely, check that no other email clients are running, and try again. On Windows, you can also restart your computer to may support the file is fully released.