• Go to the Office 365 admin portal
  
• Open the SharePoint admin center
  
• On the left choose user profiles
  
• Manage User Profiles
  

Manage User Profiles

• Find the user you would like to have access to
  
• Open the item menu of the user
  
• Manage site collection owners
  

Administrative Access OneDrive for Business Manage User Profiles Item Menu

• Add your administrator account
  

OneDrive for Business Site Collection Admin

After executing the steps above, you will see all files and folders for the current user:

Administrative access OneDrive for Business

http://www.eugenedauphin.com/2015/01/get-administrative-access-onedrive-business-environment-user/