Many countries have been ruled by an oligarchy, with South Africa during apartheid being one such example (where race was used as the basis for an oligarchy), but Russia and America (where wealth is often the determining factor for oligarchical rule) are both oligarchies. Many countries may be oligarchical in nature, despite having a visibly different official political system.
An oligarchy is a form of government that is controlled by a small elite, which are either given power by law (a de jure oligarchy), or are effectively able to buy their way into power using wealth, although by law all people are treated equally (a de facto oligarchy).
Oligarchies are not always based on wealth alone, as early 20th century South Africa has shown. Almost any differentiation can be used as the basis for an oligarchy, including blood ties, religion and race.
History shows that oligarchies are often tyrannical in nature, relying heavily on public servitude or indifference to continue and maintain their existence.
De facto oligarchies are generally more common than de jure, especially given the huge wealth and power some institutions can wield (international finance corporations, for example). This has led, in some countries, to what is known as a corporatocracy, or corporate oligarchy.