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How to Import OST to Office 365? A Comprehensive Guide

Author : Ortan Parkar | Published On : 08 Jun 2026

With businesses moving rapidly toward cloud-first environments, Microsoft Office 365, now Microsoft 365, has become the go-to platform for corporate communication. However, managing legacy data during this transition can be tricky. One of the most common challenges administrators and users face is dealing with OST, i.e, Offline Storage Table files. If you have an unconfigured, orphaned, or backup OST file and need to get its emails, contacts, and calendars into the cloud, you are in the right place. This step-by-step guide walks you through both manual techniques and a seamless automated solution for importing your OST files directly into Office 365.

Understanding OST Files and the Need to Move to the Cloud

An OST file is a local cache created by Microsoft Outlook to save a copy of your mailbox data for offline use. It is tightly bound to the specific Exchange account and the specific computer where it was created. 

While OST files are great for offline productivity, they become highly problematic when:

  • The original Exchange Server crashes or is decommissioned.
  • An employee leaves the company, and you are left with an "orphaned" OST file.
  • Your organization is migrating from an on-premises Exchange setup to the cloud-based Office 365 environment.

Because an OST file cannot be opened directly by any other Outlook profile or easily dragged and dropped into Microsoft 365, a proper migration strategy is essential.

 

Methods to Migrate OST into Office 365

There are 2 main approaches that are widely used to import OST into Office 365:

  • Manual Method

  • Automated Way

1) Manual Method for Importing OST to Office 365 

Microsoft does not provide a direct feature to "upload an OST file" to Office 365. Therefore, the manual method requires a two-step workaround: first, you must convert the OST file into a PST file using the original Outlook profile, and then import that PST file into Office 365.

Step 1: Export OST to PST via Outlook

  1. Open Microsoft Outlook on the machine containing the configured profile.

  2. Go to the File menu, click Open & Export, and select Import/Export.

  3. In the Import and Export Wizard, choose Export to a file and click Next.

  4. Select Outlook Data File (.pst) and click Next.

  5. Choose the specific mailbox folders you want to export and click Next.

  6. Browse to select a save location for the new PST file, choose your preferred duplicate option, and click Finish.

Note: This step only works if Outlook is still connected to the original Exchange account that created the OST file.

Step 2: Import the PST File into Office 365

Once you have the PST file, you can upload it to Office 365 using Outlook's Import/Export wizard again (by configuring your new Office 365 account in Outlook) or by using the Office 365 Import Service via network uploading in the Microsoft Purview compliance portal.

Limitations of  Manual Migration

While the manual method is free, it comes with heavy technical baggage that makes it impractical for many scenarios:

  • If the Exchange server is down or the mailbox has been deleted, the manual method completely fails because you cannot open or export the orphaned OST file.
  • OST files are notoriously fragile. If your file is even slightly corrupted, Outlook will refuse to export it, resulting in data loss.
  • Exporting large mailboxes and then manually uploading them to the cloud takes hours or even days and demands constant human monitoring.
  • If you have 50 user OST files to migrate, you have to repeat this entire tedious process 50 separate times.

2) The Automated and Risk-Free Way Using MailsDaddy

To bypass the strict limitations and risks of the native manual approach, professional IT administrators rely on specialized utilities. The MailsDaddy OST to Office 365 Migration Tool is a dedicated software designed to migrate data directly from any OST file to Microsoft 365 without requiring an active Exchange server or complex server-side configurations.

Key Features of the MailsDaddy OST to Office 365 Migration Tool

Using a dedicated utility turns a highly complex technical task into a few simple clicks. Some of its standout features include:

  • It seamlessly reads and migrates data from orphaned, disconnected, or completely inaccessible OST files.
  • The tool automatically scans and fixes minor to severe corruptions within the OST file before uploading it.
  • You can upload multiple OST files to multiple Office 365 user accounts simultaneously using a CSV mapping file.
  • It preserves the exact folder structure, read/unread status, inline images, attachments, and meta-properties like To, CC, BCC, Date, and Time.
  • You can filter data by specific date ranges or item types (e.g., migrating only emails from the last year while leaving calendars behind).

Step-by-Step Process to Import OST to Office 365 Using MailsDaddy

The migration process is incredibly straightforward and can be broken down into just a few simple steps:

  1. Download, install, and open the MailsDaddy OST to Office 365 Migration Tool on your Windows computer.

  2. Enter a valid License key; otherwise, continue with the demo version (20 items/folder).

  3. Click on the Add File button. You can choose to add a single file or search an entire directory to find hidden OST files automatically.

  4. The software quickly loads the file and displays a complete, tree-like preview of all internal folders, emails, and attachments.

  5. Click the Migrate button from the top menu bar 

  6. Choose between migrating to a single user mailbox or using Admin Impersonation for multiple mailboxes. Enter your Office 365 credentials securely.

  7. Set any date filters or folder exclusions if necessary, then click Export/Migrate. The tool will safely stream your data directly into the specified Office 365 cloud environment.

Note: In the Demo version, you migrate 20 items per folder

Conclusion

Importing an OST file into Office 365 doesn't have to be a stressful task. If you have an active Outlook profile and a small, healthy file, the manual approach of converting to PST and uploading can get the job done. However, if you are dealing with an orphaned file, severe corruption, or a batch of multiple mailboxes, the native method falls short. Utilizing a professional tool like the MailsDaddy OST to Office 365 Migration Tool keeps your data safe, eliminates downtime, and ensures a seamless transition to the Microsoft cloud.