OPEC countries have nationalized their oil fields. They decide how much each member country will produce. They put their oil on the same market with everyone else. If they flood the market the price goes down if they decrease production the price goes up.
They really want to sell us finished product and if we don't build some refineries in the US, OPEC will have control of the price of gasoline, not just oil.
This site has a lot of good information.
Oil Price History and Analysis
The Prize by Daniel Yergin , is the best history of international oil exploration I've read.