It is easier said than done. In order to stop trading in US dollar, there must be a viable alternative trade medium. The minimum requirement of said medium is the ability for said country to resist an US invasion, which pretty much cross out 90% of the currencies in the world. This leaves European Euro, Chinese RMB, Russian Ruble and possibly India Rupee and Brazilian Real. The later two's military capacity, while decent, doesn't quite match up to the likes of US, but the nations are big enough to potential make invasion costly. Russian Ruble is out because frankly, Russia economy is terrible. This leaves European Euro and Chinese RMB. The former is severely undermined with the debt crisis and the latter is only slowly increasing its influence.
Of course, in order to wrestle OPEC away from US, there must also be someone who is powerful enough to force US away from middle east, which is currently beyond the ability of any nation in the world. (In a few decades, this might be different)