What is the structure of an oligarchy and what are some past or present countries with an oligarchy?

Answer 1

An oligarchy is "rule by the few". It does not specify how those few are defined, though traditional methods are by membership in some organization, birth into a specific set of families (or social class/caste), or wealth.

In modern usage, it generally EXCLUDES forms of government like a monarchy or Dictatorship, in that an oligarchy generally uses some form of group decision-making. For the most part, the modern oligarchy is structured much like a modern republic or democracy, but "voting" is extremely limited.

Ancient Oligarchies would include the Holy Roman Empire (where Emperors were selected by a very limited number of Electors) and Japan under the Shoguns (where the Emperor was a figurehead and actual rulership was a competition between various Great Families).

Modern Oligarchies are harder to define, though I would personally classify many African nations as practical or de-facto oligarchies, in that while all citizens nominally can vote, the reality is that only a very limited number of families supply all the politicians, and control all the wealth and power in the country.