Beyond what jcranmer wrote, if OPEC tomorrow priced oil in renminbi or rubles, China or Russia would be pleased because it would simplify transactions for them. But that decision isn't up to China or Russia; it's up to the various OPEC member states, who all want dollars and not Chinese or Russian funny money. And why should they do otherwise, when the Chinese and Russians themselves don't want their own currency, much preferring to hold dollars?