What are the characteristics of an oligopoly?

An oligopoly is defined as a market structure where the market is dominated by a few large firms. Within the oligopoly, there is mutal interdependence, where firms base their prices and marketing strategies based upon the likely response of other firms in the oligopoly. There is also non-price competition, where advertising and other means are used to distinguish a firm's goods from another to try and sell their goods with the increased competition which exists in an olipopoly. There are also strong barriers to entry where it is difficult for new firms to enter the market.