Written by Jennifer Walsh. Reviewed for technical accuracy by Shubham Dixit, Independent Expert in Email Forensics and Data File Conversion.
Shubham is an independent external reviewer and not an employee of PCDOTS.
Quick answer. An OST is a local cache of a mailbox that lives on the server, and neither Outlook nor Office 365 can import an OST file. If your account still works, the mail is already in Office 365, so you just add the account. Only an orphaned OST that Outlook cannot open needs extracting, with a tool, into 365 or a PST.
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What an OST really is?
This is the part that changes everything, so it is worth a moment. An OST is not your only copy of anything. It is a local cache that Outlook builds of a mailbox that already lives on a server, Exchange or Office 365, so you can read your mail offline. The server holds the real mailbox, and the .ost on your disk mirrors it. That is why neither Outlook nor Office 365 has an import OST button the way they do for PST. So the honest question is not how to import the OST, it is which situation you are in.
The OST mirrors the server mailbox, so it is rarely the only copy of your mail.
“Half the time someone asks me to import an OST into 365, the mail is already there. The OST was only ever a cache of the server, so reconnecting the account brings it all back with nothing to migrate. The case that genuinely needs work is the orphaned OST, where the profile is gone or the mailbox was deleted and Outlook simply refuses to open the file. There is no native route for that one, and reading the raw .ost straight into 365 is exactly where a dedicated tool earns its keep.” Shubham Dixit, Data Migration reviewer (draft, pending approval)
If the account still works
Start here, because it is the most common case and it is free. If you can still sign in to the mailbox, the mail in your OST is already sitting in Office 365, because the OST only ever copied it down from there. So there is nothing to import. Add the account to Outlook, let it sync, and everything reappears. If you are an admin moving people from an on-premises Exchange server to 365, that is a server-side migration from the Exchange admin center, again not an OST import. In both cases the OST file on the old machine is just a leftover cache you can leave behind.
If you need it as a file, export a PST
Sometimes you want the mail as a portable file, for an archive or to hand to an admin. If Outlook can still open the account, you do not touch the .ost directly, you let Outlook write a PST. Go to File, Open and Export, Import / Export, choose Export to a file, then Outlook Data File (.pst), and save. From there you upload that PST into Office 365, either by dragging it into the 365 mailbox inside Outlook, or through the admin Import service with a network upload. The one condition is that the profile still works, because this whole route runs through Outlook. If Outlook cannot open the mailbox, you are in the orphaned case below.
If the OST is orphaned, a tool
Here is the situation with no free answer. The profile is gone, the account was deleted, or the person has left, and when you double-click the .ost Outlook refuses to open it, because an OST is tied to the profile that made it. Microsoft gives you no way in. A dedicated tool reads the raw .ost directly, no matching profile required, and writes it out, either straight into an Office 365 mailbox over a sign-in, or to a PST you upload. The PCDOTS OST Converter does this on Windows, opening an orphaned .ost and sending it to 365, with a preview and a free version that handles the first ten items per folder so you can confirm it reads your file before you buy.
A live account needs no import, an orphaned OST is where a tool comes in.
The tool route, an orphaned .ost read straight into an Office 365 mailbox.
Here are the tool steps with the screens.
Step 1. Install and run the software on Windows, then open the menu.
Step 2. Choose OST files and add the orphaned .ost so it loads.
Step 3. Open Export and choose Office 365 as the destination.
Step 4. Sign in with the Office 365 account and start the upload.
For related routes see our guides to move Outlook OST data files, import a PST to Office 365 for the upload half, and export an Office 365 mailbox to PST for the trip back.
The methods compared
Match the route to the state of your account.
| Situation | What to do | Good to know |
|---|---|---|
| Account still works | Add it, it syncs | Mail already in 365, free |
| Want a file copy | Export a PST, upload it | Needs a working profile |
| Orphaned OST | A tool reads it into 365 | Windows, no Outlook needed |
Frequently asked questions
Can Outlook or Office 365 import an OST file?
No. There is no import OST feature, unlike PST. An OST is a cache tied to its profile, so you reconnect the account or extract the file with a tool.
My account still works, how do I get the OST mail into 365?
You already have it. The mail was cached from the server, so add the account to Outlook and let it sync. Nothing needs importing.
How do I turn a working OST into a file for 365?
In Outlook use File, Open and Export, Import / Export, Export to a file, and choose Outlook Data File (.pst). Then upload that PST into 365.
My OST is orphaned and Outlook will not open it, now what?
A dedicated tool reads the raw .ost without a matching profile and writes it into an Office 365 mailbox, or to a PST you upload. That is the case software is really for.
How does an admin migrate on-premises Exchange to 365?
Through a server-side migration in the Exchange admin center, cutover, staged or hybrid. That moves the mailboxes directly and does not involve OST files.
Do I need Outlook installed to move an orphaned OST?
No. A tool that reads the .ost directly and signs in to 365 does the job without Outlook, which is the point of using one for an orphaned file.
What to actually do
So skip the search for an import button, there is not one, and pick by your situation instead. If you can still sign in, add the account and your mail is already there in 365, at no cost. If you want a portable copy, let Outlook export a PST and upload that. And if the OST is orphaned, the profile dead and Outlook refusing to open it, that is the one case where a tool that reads the raw file into 365 truly saves you. Read your situation first, and the OST stops being a puzzle.
