I am not sure how the sync between AD and sharepoint online users profile service work. let say i modify the sharepoint user profile service property named Manager for a user, and inside AD this user does not have any manager specified. Then will the manager value i specified inside the sharepoint user profile service get overridden to match the value inside AD (which is null or empty)? I mentioned the Manager property as an example, but i am referring to the whole sharepoint online user profile service's properties.
in our case we use the DirSync for our office 365 users. Regards