Going to the WTO to break up OPEC (or uselessly punish them in some way) would seem hypocritical. The U.S. imposes a tariff on incoming gasoline to encourage domestic refining, meddling in the same market OPEC is.

Of course, appearances are beside the point. The U.S. could not get the WTO to take any substantive action against OPEC, since so much of the world depends on these countries. If we tried suing them in the U.S., they'd laugh at us and never pay a nickel. (Of course, we could steal it out of their investments here if we had an urge to immediately scare off all foreign capital and plunge ourselves into a crisis.)

The U.S. simply does not have the diplomatic or economic power to confront OPEC. WTO countries have commercial ties (not just oil, investment) to OPEC, and would vote against member countries at their peril. By the way, Iraq is in OPEC. Care to almost ensure defeat in the war by antagonizing it?