Wednesday 13th August 2025

Here are the complete steps to upload OST file to Office 365 with attachments. Read the post and know the detailed OST to Office 365 migration process.

Whether you recently switched to Office 365 or your Outlook suddenly stopped, then you must be aware of OST file. This file holds all your emails, attachments, contacts, calendars, and other mailbox data. If you are planning to upload OST emails to Office 365, then you can’t just drag and drop them.

Many users face the issue when they plan to import OST file to Office 365 mailbox. In this post, we will discuss the best solutions to complete the migration process.

What is an OST File? It is an offline data file that stores Outlook’s complete mailbox data. OST is created when you are connected to Exchange or any IMAP accounts. It will help user to access their emails offline, and the changes you make will be synced when you are back online. But with the OST file, there are a few issues –

  • The OST file is linked to a specific Outlook profile.
  • It is not possible to upload OST to Office 365 and other email accounts.
  • In the absence of the original account, the OST file becomes an orphan.

Why Need to Import OST File to Office 365?

Here are a few situations where users want to upload emails from OST file to Office 365 mailbox. Some of the common ones are –

  • Your company moved from Outlook or Exchange to Office 365.
  • If your current Outlook account has been deleted or stopped working.
  • You need to get emails from an old OST file into your webmail account.
  • Switching to a new computer with an old email, then importing OST to Office 365 will be helpful.
  • You want to secure your emails in the cloud that can be easily accessed.

How to Upload OST File to Office 365 Account?

Although Office 365 doesn’t import OST file in its interface, there are a few workarounds. Here in this post, we will discuss some of them. Here we will discuss one manual and one automated way to complete the OST to Office 365 migration process. Let’s begin with.

Method 1: Upload OST Emails to Office 365 Manually?

To manually import OST emails into Office 365, you first need to convert them into a compatible format, i.e., a PST file. Here are the steps to complete the uploading –

Step 1 – Export OST Email to PST

If the OST file is created by the same Outlook you are using, then follow the steps to export your emails. If the OST file is orphaned, then you need a tool to convert non-accessible OST file into PST format.  Follow the steps –

  1. Open Outlook on your computer.
  2. Go to File → Open & Export → Import/Export.
  3. Choose Export to a file → Outlook Data File (.pst).
  4. Pick the email account or folders you want.
  5. If you want to include subfolders, you can choose them.
  6. Choose where to save it and click Finish.

Step 2 – Send Your PST File to Microsoft

Now you have a .pst file, it’s time to move it to your Office 365 mailbox. To import the data file, there are two ways –

Option 1 – Upload Online

  1. Use Microsoft’s AzCopy tool to upload the file to their cloud.

Option 2 – Send a Hard Drive

  1. Copy the file to a password-protected (BitLocker) drive.
  2. Ship it to Microsoft. They’ll put it online for you (extra cost).

Step 3 – Set Up Your Office 365 Mailboxes

  1. Create Office 365 accounts for each person.
  2. Give them email licenses.
  3. Match each PST file to the right mailbox.

Step 4 – Import Emails into Office 365

  1. Make a small CSV file that tells Microsoft which PST belongs to which mailbox.
  2. Upload it and start the import.

Done! Here, you manually upload OST emails to Office 365 mailbox, but you can face a few hurdles.

  • It is a time-consuming process.
  • Requires technical knowledge to migrate emails.
  • No direct option to import OST to Office 365 mailbox.
  • Risk of data loss.
  • Risky to import a large OST file.

How to Import OST File to Office 365  automatically?

With the help of the BLR OST Converter tool, one can directly upload their OST file data into Office 365 mailbox without any hassle. Most importantly, you can import emails from corrupted, damaged, or inaccessible OST file. After you load the OST file, you can choose the email folders and import them only. It has a simple and user-friendly interface that is suitable for both technical and non-technical users.

Download the demo version of the software and learn about its working process. The trial one will import 15 emails from each folder.

download BLR OST Converter software

Steps to Upload OST File to Office 365 Mailbox

1. Open the BLR OST File Converter tool on your Windows.

2. To start the process, click on Add file and browse the data file.

Add the OST fil;e to upload into Office 365

3. Choose the folders you want to migrate.

Choose email folders to upload to Office 365

4. From the multiple export options, choose Office 365.

Import OST to Office 365

5. Click on the Sign in button, and it will direct you to the  Office 365 login page.

Upload OST File to Office 365

6. Then enter your Office 365 login details and Sign In. After authenticating, you will be redirected to the tool, hete click on the Export button.

Import OST File to Office 365

Done! Now the tool will start importing OST emails to Office 365 mailbox. Noted that, use the app password while synchronizing with the Office 365 mailbox.

Why Use the BLR OST Converter Tool Over Manual?

  • Various outstanding features of this tool can make uploading an easy task for you.
  • Effortlessly import OST emails to Office 365 with attachments.
  • Maintain email folders and sub-folder hierarchy and structured.
  • Option to convert OST to PDF, CSV, MSG, MBOX, and other file formats.
  • Filter to upload emails from a specific date into Office 365 mailbox.
  • Option to delete duplicate emails from the OST file.
  • It also upload OST file to Gmail and many other webmail accounts.
  • Works on Windows 11, 10, 7, 8, and other versions.
  • Upload corrupted, damaged, or orphaned OST file without any hassle.
  • A free demo version is also available to analyze its working process.

Conclusion

In this article, we discuss the two solutions to upload OST file to Office 365 mailbox. You can either use the manual method or use the direct approach. Although the native method can complete the OST to Office 365 migration, it is not error-free. If you want and hassle-free upload, then we suggest trying the BLR Tools. Now you can choose the best solution for you.