Dude, you need to read KGB's post again. The guy was nice enough to lay it out for you without flaming your ass and you seem to be ignoring. And as Szeven said, if you have no clue what the ETF is all about you will have your lunch eaten by those who do , have your ass summarily handed to you , and you will be sent on your way.
Today was exceptional, not a standard expiration on the crude. With the events of the last week, the dollar getting smashed, and some idiots being way too short going into to expiration they got the living shit squeezed out of them and taught a lesson. That violent price movement happened on fairly low volume and was basically confined to the Oct contract.
As was pointed out , the USO holds a variety of futs contracts and they are likely not stupid enough to be holding a bunch of Oct contracts about expire, for the very reason you witnessed today. The USO is a good proxy for crude, particularily from early to mid, late month, but when contracts are rolling it can de-couple somewhat. It is an ETF that is proxy, not the commodity it self.