The distinctive feature of an oligopoly is interdependence. Oligopolies are typically composed of a few large firms. Each firm is so large that its actions affect market conditions. Therefore the competing firms will be aware of a firm’s market actions and will respond appropriately.
This means that in contemplating a market action, a firm must take into consideration the possible reactions of all competing firms and the firm’s countermoves. It is very much like a game of chess or pool in which a player must anticipate a whole sequence of moves and countermoves in determining how to achieve his or her objectives.
For example, an oligopoly considering a price reduction may wish to estimate the likelihood that competing firms would also lower their prices and possibly trigger a ruinous price war. Or if the firm is considering a price increase, it may want to know whether other firms will also increase prices or hold existing prices constant.
This high degree of interdependence and need to be aware of what other firms are doing or might do is to be contrasted with lack of interdependence in other market structures.
Interdependence is good for an oligopoly as there are only a limited number of firms in the market, so competitive pricing ultimately benefits the consumer.