In this new article about how to add a secondary administrator to OneDrive for Business, we will review how to perform this action through the SharePoint Online Administration Center:
First of all, browse SharePoint Online (SPO) Admin Center and click on User profiles. In the user profiles page, just click on Manage User Profiles.
In the Manage User Profiles page, just search for a specific user and display administracion options available for managing that user profile. Click Manage site collection owners option.
In the site collection owners window that is displayed, just add / delete secondary administrators for the user’s OneDrive.