There is an inverse relationship between the U.S. dollar and oil prices. Oil trades in U.S. dollars all over the world.
Make this website one of your favorites: http://www.marketwatch.com/investing/index/dxy/charts
The chart will often tell you why oil prices are up or down on any given day.
Oil Prices - May 13
Oil Prices - May 13
Dan Steffens
Energy Prospectus Group
Energy Prospectus Group