The fact that most oil *must* be traded using US dollars is a HUGE advantage to us. It means that the dollar is worth much more in the world's economy. And countries like Saudi Arabia buy a lot of military hardware from the US, using all the US dollars they have.
Remember the so-called "
Axis of Evil"? The three countries on that list were Iran, Iraq, and North Korea -- they refused to use the US dollar and were trading oil in their own currencies. That was just about the only thing that all three had in common...
Here's a video called "Robert Newman's History of Oil" that you might find helpful: