In the Cournot theory, as more firms enter the market, lower prices and profits result, leading closer to pure competition.  
The most pessimistic predictor of behavior in it is the Bertrand theory.  The presence of strategic behavior makes actions in it significantly less predictable than in its two alternatives.  In the Cournot theory, as more firms enter the market, lower prices and profits result, leading closer to pure competition.  Cartels may form in, FTP, what type of market with substantial economies of scale, high entry barriers, and few firms?
The most pessimistic predictor of behavior in it is the Bertrand theory.  
Oligopoly