Also, worth mentioning, when you go to delete a user in O365, it should prompt you to change the ownership. Otherwise, if the OneDrive already has a alternate assignee it will delegate to them. This process emails that user or new assignee/owner of the drive an email.
Don't know if this will help, but I posted the steps I have found useful below.
After you have signed in, to the admin portal
(1.) you will locate the "Users" section
(2.) then go to "Active users"
(3.) Then search for the user your looking to move/save/or
change ownership of the OneDrive data from.
(4.) Select the check box to the left of the users "Display name"
(5.) Block the termination user. At the top of the new kick out window, click on the button (under the users name) labeled "Block sign-in"
(6.) Then you can scroll down through the user's option till you get to the "OneDrive Settings"
Skip "Access - copying files" for now, we want to kick them from all connections to the OneDrive.
(1.) Change setting: External Sharing
- Turn this off
(2.) Sign-out (make sure the user's AD account and access has been disabled?)
- Click "Initiate"
(1.) Select all or some of the documents that are needed
(2.) Then click "Download"
(3.) It will download a .zip file to which can be moved or store somewhere if needed.