Pretty simple really; a monarchy is rule by one whereas an oligarchy is rule by some. In an absolute monarchy there is a single person who is the head of state and the ultimate authority in government. The best current example of an absolute monarchy is the Vatican Holy See, with the Pope as the monarch. In an oligarchy the state is controlled by a small (usually) group of individuals (usually the elites), one of whom will normally act as the head of state but authority rests with the ruling group. There are no formal oligarchies that I am aware of; however, China is a practical oligarchy with the Communist Party's top brass having effective control of the state. Many claim that Russia is also a practical oligarchy with group of wealthy elites effectively controlling the state.
