Integrating Office 365 with Active Directory, not Single Sign on

For example for a company who already uses Office 365 for mailboxes, and already has Active directory for file sharing use on a Windows server 2012? 

Is it fairly straight forward?

The client machines won't be bound into the Directory server so we wouldn't be looking at an ADFS server, literally just so the Active Directory and Office 365 usernames/Passwords are the same.

Thanks