Edit ownership of groups

[Leder DKFU DKFU]

I was just made global admin of the entire organization. 

Under "documents", theres a person who are the owner of several folders. I have deleted this person as user, as he no longer is employed. Though still remain as owner.

So far I haven't figured out how to delete him. How?

[#2]

You cannot, the usernames stay part of the list items indefinitely even after manually removing the users from the site collection / Office 365 environment. You would have to basically recreate the folders them use move to commands to move documents into the folder to get rid of the name if you have too. You could also use other 3rd party tools to basically Migrate content onto itself and remove the users that do not exist anymore in that process.