Recent news (the President revoking the executive ban on offshore drilling, and his calling on Congress to repeal their part of the ban) has convinced some of the market that new supplies will be forthcoming (at various time-scales.) The result is that some (but not all) buyers are no longer willing to pay as much as before for oil (or for future contracts to buy oil). It also convinced some (but not all) producers to be willing to accept a lower price for oil (or to sell future contracts.) This is what actually brought down the market price.

Cartels like OPEC can collaborate to post a uniform “willing to sell” price, but it’s still market forces (via the exchange) that combines this with other sellers’ prices and buyer’s prices to determine the actual amount of money handed over for a barrel of oil.