When you use Microsoft Outlook offline, it makes OST files that hold the whole mailbox folder, which contains emails, contacts, calendars, and more. But a user can't simply get to it from anywhere else but the Microsoft Outlook profile that made it in the first place. As a result, customers are always looking for a safe way to keep and access their OST file database without any restrictions. Now that we know for sure that Office 365 is one of the safest places to store a database, it's no surprise that most OST users prefer to back up their OST files to the Office 365 app. The Office 365 app has a lot of benefits for everyone who uses it. But now the user doesn't know how to back up their OST file on the Office 365 cloud service.

You don't have to worry anymore. Just read this whole post, which will show you all the ways you can back up your OST files and move them to Office 365 without losing any files. So, let's read the post...

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Ways to Move OST Files to Office 365

Well! There are two best ways to figure out how to back up an OST file in an Office 365 account. The first is the manual method, and the second is the professional method. We will talk about both methods in this section so that you can choose the one that works best for you.

Method 1: How to Upload an OST File to Office 365 Manual

You have to go through both conversion rounds for this method to work. In the first step, users have to change the OST database to PST format. In the second round, they have to move all the PST data into an Office 365 account. Let's go through the steps of conversion below:

Round 1: Change an OST file to a PST file. 

Open the MS Outlook program on your computer.

Go to the File menu and choose Export to a File from the list of options.

Press the next button now to move on.

You need to choose the Personnel File Folder (PST) at this stage and then click on the Next tab again.

Select a folder that you wish to export and check the option for "Include subfolders."

In the last stage of this round, type in the path to the folder where you want to save the exported file and click the Finish button.

Important: You need to be connected to the Exchange Server environment for the procedures above.

Round 2: Put PST Files in Your Office 365 Account 
Follow the second phase of the manual method, which lets you upload all of your PST files to the O365 Cloud app.

To learn more, read How to Import a PST File to Office 365.

Method 2: A Professional Way to Upload an OST File to Your Office 365 Account 

You can easily download one of the excellent OST Converter programs that experts recommend using this approach. There are a few steps to converting this solution that make it easy to save backup OST files to Office 365 accounts right away.  Softaken OST to Office 365 is a transfer of mailbox data from OST files directly into Office 365 cloud accounts. It helps move emails, contacts, calendars, tasks, and other mailbox items without needing Outlook or Exchange Server configuration. The software works by scanning offline OST files and uploading selected folders into the desired Microsoft 365 mailbox with just a few clicks. The tool supports both healthy and orphaned OST files of any size and maintains the original folder structure during migration. It runs on all modern Windows platforms, including Windows 11 and 10. 

Key Features

Direct Migration of OST files to Office 365 without Outlook or Exchange

Preview Mode to verify emails, contacts, calendars, and other items before export

Selective Folder Export – choose specific folders or the entire mailbox for migration.

Supports Orphaned & Corrupt OST Files with data integrity maintained

Maintains the original structure and metadata (To, CC, BCC, Subject, Attachments).

Handles Large OST Files smoothly without size limitations.

Batch Migration Supported—move multiple OST files at once.

Compatible with Outlook Versions 2003 to 2021 and Microsoft 365

Free Demo Available with limited export

Technical Support Provided for installation and migration assistance

Let's go through the step-by-step guide below:

Download and install the tool on Windows.

Select OST file (.ost) (Single or batch mode).

Select “Office 365” in the Export option.

Enter O365 account credentials and select the Target folder.

Click on “Start Uploading” → Start migration.

Conclusions 

The piece above talked about two ways to back up your OST file in your Office 365 account. You can now use any of the methods to back up your OST file in Office 365. Well! If we talk about both methods, we can see that the manual solution is too long and dangerous. On the other hand, professional techniques have a lot of benefits and are easy and quick to use. Some of these features are the ability to back up batch files, use advanced filters, and save reports. You can learn more about this method by downloading the free trial version of OST to Office 365 Converter and utilizing it to accomplish the whole conversion on any Windows OS platform.