Oligopolies behave like monopolies

Oligopolies may give the appearance of competition, but generally industries with few players in repetitive games default towards cooperation. Cooperation entails limiting supply and raising prices. Research by the OECD shows that cartels and price fixing is widespread in oligopolies. Most cases involve industries with four major players. Many oligopolies don't even need to fix prices illegally and can "signal" their intentions to competitors. This is widely understood and covered in economic literature.