It isn't only OPEC.
The US dollar shaved at least 20%+ off the price of oil, and likely instigated this crash (the world was swimming in oversupply of oil for quite some time before the crash).
The US dollar began skyrocketing, due to the end of QE, at exactly the same time oil began crashing. Not a coincidence, given oil is priced in dollars and the US dollar is on its greatest one year run in decades.
Look at July 1st