If you opened your Outlook folder and found an .ost file sitting next to an .nst file, you are probably wondering which one matters and whether either is safe to delete. This guide explains Outlook NST vs OST in plain words, what each file holds, and what changes in new Outlook.
Quick Answer: An OST file is the offline copy of your own mailbox that classic Outlook keeps on your PC when it connects to Exchange or Microsoft 365. An NST file is the local copy of the Microsoft 365 Groups you belong to. Both are caches, Outlook rebuilds them from the server, and new Outlook for Windows uses neither.
Outlook NST vs OST: What Is the Difference?
The difference between NST and OST in Outlook is simple: the OST holds your mailbox, and the NST holds your Microsoft 365 Groups. That is the whole difference in one line, and the table below puts it side by side.
| OST file | NST file | |
|---|---|---|
| Full name | Offline Storage Table | Not spelled out in Microsoft’s own pages |
| What it holds | Your mailbox: mail, calendar, contacts, tasks | Shared mailbox and calendar of the Microsoft 365 Groups your mailbox joined |
| How many | One per mailbox (account) | One per Microsoft 365 mailbox, covering all its Groups |
| Where | %localappdata%\Microsoft\Outlook | Same folder as the OST |
| Needs a backup? | No, the server has the data | No, Outlook recreates it |
| New Outlook for Windows | Not used | Not documented |
Both show up as “Outlook Data File” in the Data Files tab, which is why people mix them up. To see them yourself, go to File > Account Settings > Account Settings > Data Files in classic Outlook.
What is an OST file in Outlook?
An OST (Offline Storage Table) is the local copy of your Exchange, Microsoft 365 or Outlook.com mailbox that classic Outlook for Windows builds when Cached Exchange Mode is on. Outlook reads from this copy, so opening a folder or searching is fast, and you can keep working with no connection. Changes sync back when you are online again.
A few things worth knowing:
- Cached Exchange Mode keeps only part of your mail offline. The setting is called “Download email for the past”, and you find it under File > Account Settings > Account Settings > E-mail, then double-click the Exchange account. Microsoft’s defaults depend on disk size: 1 month on a disk of 32 GB or less, 3 months up to 64 GB, and 12 months above that.
- The OST is tied to the profile it was created for. You cannot copy it to another PC and use it there. Outlook just builds a new one.
- Classic Outlook limits an OST (and PST) to 50 GB by default, with a warning at 47.5 GB. If you ever see “The message store has reached its maximum size”, this is the limit.
- If an OST is corrupt, the Inbox Repair tool (SCANPST) will not help with an Exchange mailbox. Microsoft’s advice is to delete the OST and let Outlook rebuild it. Close Outlook first.
The OST file location is the same folder for most setups (%localappdata%\Microsoft\Outlook), and the Data Files tab shows the exact path. The difference between PST and OST is that a PST is a file you create and keep, for archives and exports, and it is not a cache. I covered that in where the Outlook PST file is and how to create a PST file in Outlook.
What is an NST file in Outlook?
An NST file is a local copy of the Microsoft 365 Groups connected to your mailbox. Outlook for Windows syncs each Group’s shared mailbox and calendar into it, for faster and offline access. There is one NST per Microsoft 365 mailbox, and it sits in the same folder as the OST. Office Watch has a good explanation of what .nst files are and whether they can be deleted.
One correction to what many pages say, including an older version of this post. NST is often described as the place where Microsoft Teams chats are stored. I could not find any Microsoft documentation that says so. What I found links the NST to Microsoft 365 Groups, and the Microsoft Q&A answer that does exist only says it marks an Office 365 (Microsoft 365) account. Microsoft has no proper public page for NST files at all, so treat any detailed claim about them, mine included, as the best available picture and not a spec.
Teams is a separate app with its own cache. If you only want to remove the Teams button or meeting link from Outlook, see how to remove a Teams meeting from an Outlook invite.
Is it safe to delete NST files?
It is safe, but pointless. Outlook recreates the NST from the online data, so deleting it does not free disk space for long, and Office Watch advises leaving it alone. You also do not need to back it up. If one is huge or looks corrupt and you want to try anyway, close Outlook completely, delete the NST (or rename it), and start Outlook again. Expect a re-sync of your Groups content.
Does new Outlook still use OST?
No. New Outlook for Windows does not create an OST for your account. It keeps its own offline cache, and Microsoft has not published where that cache lives. Observers have found it under %LocalAppData%\Microsoft\Olk, but that is an observation and not documentation. Offline access is on by default and users can change it in Outlook settings. Admins can block it with the OfflineEnabledWin setting in an OWA mailbox policy (Office 365 IT Pros has the details on offline access in new Outlook).
So the practical result is simple:
- Classic Outlook for Windows with Exchange or Microsoft 365: OST, and an NST if you are in Groups.
- New Outlook for Windows: no OST, and I found nothing documented about an NST.
If you are not sure which one you run, here is how to switch between new and classic Outlook. PST files can be opened in new Outlook with conditions (classic Outlook installed with the same bitness, a Microsoft 365 subscription, and not the ARM version), as described on Microsoft’s Outlook data files page.
How can I convert an NST file to a PST file?
You normally do not need to. The NST is only a cache of data that lives in Microsoft 365, so there is nothing in it to rescue. I found no built-in Microsoft way to convert it. Third-party converters exist, but for a cache that Outlook rebuilds for free, I would not pay for one. If you want the Group conversations kept somewhere else, use a mailbox export from the server side instead of the local file. Your admin can handle that.
If your question is about a shared mailbox you no longer need in Outlook, that is a different thing. See how to remove a shared mailbox from Outlook.
Which one do you actually need?
You do not choose. Outlook creates both on its own, depending on the account and the client. If you are an admin or a user managing disk space, the OST is the one that grows with your mailbox and the sync window, so that is the setting to adjust. Leave the NST alone.
If Outlook is eating disk space, look at the OST and the sync window first. If you only came here because the NST file looked odd, you can stop worrying about it.
