Both parties can affect the oil market in different ways. Speculators are very influential in manipulating the oil price artificially. Without real oil involves in the transactions (paper oil), speculators basically take advantage of any position in the arbitrage activities.
OPEC, on the other hand, affects the real oil market by engaging in the supply activity. It is reported that OPEC has come to its optimum production capabilities. This means OPEC will need longer period to increase oil supply.
In contrast, speculators can be very dominant in affecting price in the short run.
In order to stabilize the oil price, it is important to stop the speculative activities altogether. After all, speculation activities only benefit a few minorities at the expanse of large majority.
At the same time, the cartel power of OPEC must be put under scrutiny as the monopoly position is also unhealthy to the market.
It is not easy to evaluate which effect is more dominant than the other. Speculative activities can be very significant when speculators heavily engage in tradings. This can be seasonal as timing is a very critical factor in speculation. on the other hand, OPEC reactions usually depend on market conditions. Apart from meeting global oil demand, political pressure can be another factor that determines OPEC production. All these contribute to the dynamic of market forces.
