http://www.americanprogress.org/issues/2009/06/pdf/health_co...
Oligopolies do tend to be correlated with monopolistic pricing, but I'm not sure they allow it in an otherwise competitive market. Most oligopolies which successfully raise prices tend to be in uncompetitive markets, e.g. cell or cable service, and the price increases may be caused by the hindered competition rather than the oligopoly itself.
Consider the case of an oligopoly in the DRAM market: when they tried to raise the price of DRAM (2001-2002), they succeeded for a short time. They also collectively lost 3.3% of market share over those 2 years and members of the oligopoly (Hynix was the first, I think) quickly broke ranks in pursuit of increased profits.
http://findarticles.com/p/articles/mi_m0EIN/is_2002_Dec_17/a...