Understanding OST Files and Office 365
An OST (Offline Storage Table) file is a local cached copy of your mailbox data (emails, calendar, contacts, tasks) from accounts such as:
- Office 365 / Exchange Online
- On-premises Exchange
- IMAP accounts (in older Outlook versions)
Key characteristics:
- OST files cannot be imported into Outlook or Office 365.
- They are encrypted and bound to the profile and mailbox that created them.
- Office 365 stores data in the cloud, so it simply recreates the OST when Outlook connects.
Important Note
- OST files don’t need backups; they are copies of server data.
- Always create a copy of your OST before working on it.
- If the OST is corrupted or inaccessible, you’ll need a reputable OST-to-PST converter.
Can You Import an OST File into Office 365?
No. Outlook does not allow importing an OST file directly. OST files are linked to a specific computer or mailbox and cannot be opened or moved manually. But some workarounds can help you import OST file into Outlook.
To migrate OST data to Office 365, you can follow these two steps:
Where is the Outlook OST file located?
The default OST file location varies by Windows version, but Outlook typically stores OST files in:C:\Users\\AppData\Local\Microsoft\Outlook\
You can also find it by going to: Control Panel > Mail > Data Files > Select your account > Open File Location.
Part 1: Export the OST File From Outlook
How to Import Outlook OST to Office 365?
Method 1: Recreate the OST Automatically
This method applies when you still have access to the same Office 365 mailbox,
- Open Outlook (desktop app).
- Go to File > Add Account.
- Sign in with your Office 365 email & password.
- Ensure Cached Exchange Mode is enabled:
- File > Account Settings > Account Settings > Select account > Change > Check Use Cached Exchange Mode.
Outlook will connect to the mailbox and rebuild a fresh OST file by syncing everything from the server.
When you already have an OST and don’t want to re-download everything
If your mailbox is huge (50GB+), you may want to reuse the existing OST.
- Close Outlook
- Navigate to: C:\Users\\AppData\Local\Microsoft\Outlook\
- Rename the OST file.
- Add the Office 365 account again
- Outlook generates a fresh OST and syncs from the cloud.
Method 2: Convert OST to PST
Use the Outlook Import and Export Wizard to export the OST file as a PST:
Note: This export method does not include message rules, categories, folder properties, signatures, custom views, or your auto-complete list.
Additionally, if the OST file is inaccessible, corrupted, or the original Outlook profile is no longer available, the manual method will not work.
- Open Outlook.
- Go to File > Open & Export > Import/Export.
- Select Export to a file > Next.
- Choose Outlook Data File (.pst).
- Select the folders you want to export.
- Enable Include subfolders.
- Choose a destination location.
- Click Finish.
Microsoft provides no built-in OST to PST tool, but recommends using third-party converters. For orphaned, corrupted, or inaccessible OST files, manual export fails.
In such cases, the safest method is to use Remo OST to PST Converter, which extracts the entire mailbox without losing metadata.
- Download and install Remo OST to PST Converter
- Choose one of the two options:
Open OST File: Automatically detects the default OST location
Find OST File: Locate the OST stored in a custom folder.
- Click Browse to set the destination for saving the converted PST.
- Start the conversion.
- Once completed, click Finish and locate the new PST file.
This ensures complete extraction of emails, attachments, calendars, contacts, and all mailbox items, even from damaged OST files.
Part 2: Migrate the PST File to Office 365
Once the OST is converted to PST, you can migrate the PST to Office 365 using either:
- Drive Shipping
- Network Upload (Azure AzCopy)
Method 1: Drive Shipping to Microsoft
Use this method if you have large PST files and a slow network.
- Copy the PST files to a hard drive
- Ship the drive to Microsoft’s secure data center
- Microsoft uploads your PST files directly into your Office 365 mailbox
This is useful for bulk migrations carried out by IT admins.
Method 2: Upload PST to Office 365 Using Azure AzCopy
This is the most common method used for migration.
- Sign in to Microsoft 365 and open Security & Compliance Center.
- Navigate to Information Governance > Import > Import PST Files.
- Click New Import Job and enter a job name
- Select Upload Your Data > Next.
- Click Show Network Upload SAS URL.
- Copy the SAS URL and save it
- Download Azure AzCopy
- Run AzCopy using the command:
AzCopy.exe /Source: /Dest: /V:\AzCopy.log /Y
After upload, complete the import job from the Security & Compliance Center.
Frequently Asked Questions
You cannot directly import an OST file into Exchange Online. OST files are encrypted and tied to the original Outlook profile. To move the data, you must convert the OST to a PST file (using Outlook export or a specialized tool) and then import the PST into Exchange Online or Outlook 365.
You can’t open an OST file directly because it’s encrypted and linked to the specific Outlook profile and computer that created it. Outlook only uses OST internally for syncing with Exchange or Microsoft 365 accounts.
Export your Outlook data to a PST file, then import the PST into Office 365 using Outlook’s Import/Export wizard or the Microsoft 365 Import Tool for larger migrations.
Conclusion
Importing an OST file into Office 365 requires a two-step approach: converting the OST to PST and then uploading the PST to Microsoft 365.
Manual export works only if the Outlook profile is accessible. For damaged or orphaned OST files, using a reliable OST to PST converter ensures complete and accurate data recovery.
