What is a characteristic of an oligarchy? A. One person makes all the rules and laws.
B. People who rule are born into their positions.
C. Someone takes over to help oppressed people.
D. Power comes from having a lot of land or money.

An informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.