Microsoft Outlook PST vs OST vs OLM -- What's the Difference?

If you've ever dug around in your AppData folder -- and, let's be honest, who hasn't done that -- you may notice a couple of different file types in the Outlook folder: .pst files and .ost files.

People who know what an Exchange server does, and who are adept at working with Microsoft Outlook are, no doubt, familiar with these two file types. However, may are not. So, here's a quick article explaining the difference between the 2 file types, and what they mean for SmarterMail users.


The PST (Personal Storage Table) file is a personal data file and is most commonly known as your "Outlook Data File". It basically contains your email messages, contacts, calendar items and other Outlook data and is stored on your local machine. It can be exported, imported, and can be used to backup and/or export items from any mail account you have. It's also not deleted by Outlook if you remove the account. You can even move items from a mail server into a PST, thereby freeing up storage space you have on that mail server -- so if your email provider has a limit to the amount of email space you have, using your PST is a good way to regain some space on their server without losing any of your data.

The PST can also be copied to other installations of Outlook -- You can even import a .PST file into Outlook for Mac! So, if you have a PST from an account, you can import it into any Outlook installation: on a second computer, on a new computer, etc. 


The OST file (Offline Storage Table), on the other hand, is an offline data file that basically mirrors a PST, but doesn't offer the same flexibility. The benefit of the OST is that it allows you to have access to your Outlook data even when you're offline: on a plane, during times when there's an internet outage, etc. When the connection comes back, the OST is again synced to the mail server, as are any changes to any data that you've made. (E.g., completed a Draft message, changed an appointment, etc.) Drawbacks, however, include the fact that OST files can only be used by the Outlook account that they were created for, and you can't re-use or move an OST to another Outlook installation. 


The OLM (Outlook for Mac) file is similar to the PST, but it's a data file created solely for Outlook for MacOS. OLM files can transfer between Outlook for MacOS installs, just like PSTs can be transferred, but an OLM can NOT be imported into Outlook for Windows. The OLM file format is strictly for MacOS. That said, there ARE ways to convert an OLM to a PST using third-party utilities. However, SmarterTools does not have any recommendations for which utility to use should this need to occur. So use these tools at your own risk. Another interesting thing is that OLM files are not created automatically by Outlook for MacOS -- they are generated when you export a mailbox and select the items you want to include with the export. (I.e., emails, contacts, calendar items, tasks and notes.) In addition, if there are multiple accounts set up in Outlook for Mac, the items selected for export to an OLM are for EACH account in Outlook -- you can not simply export, say, the contacts for Account 1 to an OLM.

Importing/Exporting OLM Files

OLM files are generally much less flexible than PST files. Therefore, there are some things to be aware of when exporting OLM files from Outlook for Mac:

  • With a PST file, you can transfer an account from one Outlook on Windows to another Outlook on Windows and have that account sync back to your mail server. You cannot do that with an OLM and Outlook for Mac: if you re-import an OLM to another Outlook for Mac install, it does not sync back to the mail server. The imported files are simply stored as local files.
  • While you can select what you want to export -- calendars, contacts, etc. -- to an OLM, whatever is exported is exported for all accounts set up in Oultook for Mac. You can't simply export the contants and calendars for one acccount.
  • In addition to the above, any local files stored in On My Mac are also exported.
  • Outlook for Mac creates, by default, a contact for any account that's set up. So, when you export an OLM, and choose to export contacts, that contact is included.
  • OLM imports only include folders that have content. If you have mail folders that are empty in Outlook for Mac, and export those folders, when you import the OLM, those folders are NOT imported.
  • When importing an OLM, it's saved to the "On My Computer" section of Outlook for Mac, and therefore stored as local files. You're not even able to select an account to import TO, you simply choose what to import and then that's added to On My Computer.