The best approach is to use the eSoftTools OST to Office 365 converter to migrate the OST file into Office 365. This software is secure and provides fast support for large OST imports to Office 365. Test the tool's quality with a free trial version, which also allows you to export the first 25 OST data items per folder.
Read More:- Migrate OST files into Office 365
Method 1: Export mailbox as a PST file
Login to Outlook with the Exchange account that was used to create the OST files. And to export the OST mailboxes to PST, follow the simple procedure:
Go to Files >Open & Export and click Import & Export.
Select Export to a file; then click Next.
Select the second option—Outlook Data File (.pst)—and click Next.
Select the folder which you want to export and click Next.
Browse the location where you want to save the new PST file and choose the options regarding the duplicate items.
This will help, J wick