Discuss the etymological origin of the term. In Greek, "oligoi" means "a few" and "polein" means "to sell." The term refers to a small group of firms or corporations that overwhelmingly dominate a particular industry.
Outline the relation of oligopoly to a term your audience might be familiar with, such as monopoly. The major difference is that an oligopoly is domination of a market by at least two companies. Make clear that there is a difference between an oligopoly that colludes openly to control an industry, as OPEC does, and one that does not, such as the four major mobile phone carrier companies.
Give real-world examples of oligopolies, such as Coca-Cola and Pepsi in the soft-drink market, or the U.S. auto industry. Part of the reason these industries become so concentrated in a few companies is because of prohibitively expensive operating costs.