An oligarchy works by effectively resting power with a small number of people. This group of people could be distinguished by being royalty, having wealth, having corporate or family ties or gaining military control.
According to Princeton University, oligarchies have been tyrannical throughout history. Aristotle was the first to pioneer the use of the term oligarchy to mean a rule by wealth, but oligarchs can simply be a group of privileged people. Some examples of an oligarchy include the Vaishali and the Polish-Lithuanian Commonwealth. Sparta could also be considered an oligarchy in Ancient Greece when it clashed with Athens during the Peloponnesian war.