Short answers, pulled from the story.
Aristotle defined oligarchy as a system where men of property hold government in their hands. He stated that wherever men rule by reason of their wealth whether they be few or many that is an oligarchy.
Robert Michels expanded on the concept of minority rule through his iron law of oligarchy theory in the early 20th century. He argued that even democracies tend to become oligarchic due to the necessity of dividing labor within large organizations.
Jeffrey A. Winters and Benjamin I. Page have described Colombia Indonesia Russia Singapore and the United States as oligarchies. They acknowledge that an oligarchy does not control all political life but focuses on wealth protection.
Cleisthenes mobilised the middle class and overthrew Isagoras in the 508 507 BC Athenian Revolution. His reforms laid the foundation for Athenian democracy after Cleomenes installed Isagoras as an oligarch following the invasion of Athens in 510 BC.
Vladimir Putin came to power in 1999 and cracked down on many oligarchs arresting several for tax evasion. By the end of the 2000s decade however Putin had created a new class of oligarchs consisting mainly of his own personal friends and colleagues.