When an employee leaves, you can disable their account and block their sign-in, and you can alter your retention policy so that their files are kept practically forever even if you do delete them. But how do you access them? OneDrive generally keeps a given user's username or password, so unless they were thoughtful enough to share literally everything in their OneDrive with another user before they left the company, don't you need to log in with their username and password in order to be able to access their OneDrive?
Nope. That's not necessary. If you don't already have the rights to access their files, you can give them to yourself by quickly hopping over to the Office 365 administration portal center.