We recently added the ability to transfer ownership to another user when deleting a user’s account from AAD from the M365 admin portal. If no entry is provided, the default will transfer to the manager. https://docs.microsoft.com/en-us/onedrive/retention-and-deletion
The manager or newly assigned owner will have the ability to sync that content or download and then move to a shared library or their own OneDrive.Kurt commented
Just migrated to O365 and ran in to this issue. Cant believe Box and the like have this but not Microsoft. It pretty much make us not want to adopt OneDrive at all until this feature is added.