Profit maximisation conditions: An oligopoly maximises profits by producing where marginal revenue equals marginal costs.
Ability to set price: Oligopolies as a group are price setters rather than price takers.
Entry and exit: Barriers to entry are high. The most important barriers are economies of scale, patents, access to expensive and complex technology, and strategic actions by incumbent firms designed to discourage or destroy nascent firms. Additional sources of barriers to entry often result from government regulation favoring existing firms making it difficult for new firms to enter the market.
Number of firms: "Few" – a "handful" of sellers. There are so few firms that the actions of one firm can influence the actions of the other firms.
Long run profits: Oligopolies can retain long run abnormal profits. High barriers of entry prevent sideline firms from entering market to capture excess profits.
Product differentiation: Product may be homogeneous (steel or oil) or differentiated (automobiles).
Perfect knowledge: Assumptions about perfect knowledge vary but the knowledge of various economic actors can be generally described as selective. Oligopolies have perfect knowledge of their own cost and demand functions but their inter-firm information may be incomplete. Buyers have only imperfect knowledge as to price, cost and product quality.
Interdependence: The distinctive feature of an oligopoly is interdependence. 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. In a PC market there is zero interdependence because no firm is large enough to affect market price.