What are the conditions for an oligopolistic market?

An oligopoly market structure is one in which there are only a few sellers of a product that is similar but may also be differentiated by brand names. The petroleum refining market, airlines, automobile manufacturers, and cell phone services are examples. Generally, the conditions are such that a larger company can be more successful due to economies of scale. That also means that the few competitors are able to monitor each other as to price and quality characteristics. Sometimes there is one firm that is a price leader, but other times there is not. Oligopolists collaborating to fix prices or output is illegal in the United States, but not everywhere. The Organization of Petroleum Exporting Countries (OPEC) is a global cartel that does collude on prices and output.