Locate the User Profile Service App (In O365 admin portal > under admin center > Sharepoint > User Profiles)
- Look up the user
- right click on their account and select Manage Site Collection Owners
If the employee leaves the company, you can add the designated employee as the primary site collection admin for access. If there's more than one designated employee that needs access, you can add the rest in site collection admins. If an employee returns, just make sure that they are set back as the primary site collection admin and the rest are removed.