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Why does Office 365 not accept OST files directly?
Can you directly import an OST into Office 365? No. The OST is a cache file which is specific to one Outlook profile and one server account. It wasn’t designed to be moved. Microsoft’s import service only accepts PST files so a raw OST will be rejected.
The file is also dependent on its original mailbox. If that mailbox is deleted then Outlook will consider the OST to be orphaned and will not open it. The same applies if the file itself is corrupted. Microsoft provide no tool to deal with either of those scenarios and that’s why people end up looking for a way to open an OST file without Outlook.
How do I find the location of the OST file in Windows 11 if I’m doing a migration? Open File Explorer and navigate to C:\Users\YourName\AppData\Local\Microsoft\Outlook. AppData is hidden so you’ll need to turn on “Hidden items” in the View menu. Copy the OST somewhere safe before you touch it.
Method 1: Convert OST to PST via Outlook, Then Import to Office 365
This will only work if Outlook is still opening your OST and the mailbox in question still exists. It’s only really suitable for one or two users.
- Open Outlook, go to File > Open & Export > Import/Export.
- Choose Export to a file, then Outlook Data File (.pst).
- Select the mailbox or folders. Tick “Include subfolders” (obviously).
- Pick a save location and carry on.
- Sign in to your Microsoft 365 account via Outlook, then go to File > Open & Export > Import/Export > Import from another program or file and select your PST.
This is the easiest way to save your Outlook OST emails as PST for free. The problem is of course size. A 30GB mailbox can take hours and Outlook may freeze part way through. The calendar and contacts will also be transferred if you select the entire mailbox. How do I transfer my OST contacts and calendar to Microsoft 365? Export the full mailbox rather than just the Inbox.
Method 2: Azure AzCopy Network Upload Using SAS URL and CSV Mapping
This is the admin route and it’s for batch uploading OST files to an Office 365 tenant once they’ve been converted to PST. It’s the way to export Outlook OST to the Office 365 admin centre on a large scale project.
- In the Microsoft Purview portal, go to Data lifecycle management and select Import.
- Create a new import job and copy the SAS URL.
- Download AzCopy and upload your PST files to Azure via that URL.
- Build a CSV file which maps each PST to a user mailbox. To import OST data into a shared mailbox in Office 365, put the shared mailbox address in the Mailbox column.
- Upload the CSV, validate it and then kick off the import.
Expect delays. Microsoft can take days to process a large job and a single typo in the CSV can bring it all to a standstill. Think of an IT team with 80 mailboxes and one incorrect file path. They’ll lose a full day tracking it down.
Both approaches have the same shortcoming in that they can’t deal with an orphaned OST. If you want to migrate an orphaned OST to Office 365 or migrate corrupted OST data to a Microsoft 365 mailbox you need a different method.
Method 3: Direct Migration with BreviSoft OST to PST Converter Tool
This is the shortest route. The BreviSoft OST to PST Converter tool simply reads the OST directly off your drive. You don’t need Outlook at all, you don’t need an Exchange login or the original account. That’s how you import an OST into Office 365 without Outlook and how you import an inaccessible OST into a Microsoft 365 account.
- Download, install and run the BreviSoft OST to PST Converter tool on Windows.
- Click Add File and select your OST from your system.
- Sacan OST file and preview the folders, emails, contacts and calendars.
- Choose save as Office 365 option from Export as Email Client feature.
- Enter your Microsoft 365 credentials and login.
- Apply date or folder filters if you only require a subset of the data.
- Click on Convert and migrate OST file to Office 365.
It’ll also work for a direct migration from OST to Exchange Online and it can export to PST, EML, MBOX and a range of other formats for various email clients. If you’d prefer to keep a local copy first then you can use the tool to convert your MS Outlook OST to PST and import later.
A quick one. A company has shut down an old Exchange server and come across 25 OST files on the laptops. They load each one up, send the data over to the corresponding Microsoft 365 mailboxes and are done in a day. Run the trial on one file first to check the outcome.
Conclusion
Microsoft won’t import an OST directly, so every route starts with conversion. Outlook and AzCopy work for healthy files and small teams. They fall short for orphaned, corrupted, or bulk jobs. The BreviSoft OST to PST Converter Tool removes those limits by migrating straight into Office 365 without Outlook. Test it on one mailbox today.
Troubleshooting and FAQ
Ans – Nothing. Your original OST remains where it is. It’s there for 30 days before you can go and delete it in case anything’s missing.
Ans – Check your CSV for incorrect file names or mailbox addresses. Make sure the PST files have completed uploading.
