Yes, OPEC does control output and tries to be a cartel, but it does not by any means control all output - it has 66% of the reserves and 36% of production, so there is still significant scope for others to sell.
Furthermore, the largest problem OPEC - and all cartels have - are members cheating. Output of the 13 OPEC states is actually higher than their "agreed" output.
Yes, it is a contributor to high prices, but it is not by any means what has driven $140 oil.