It's difficult to know (at least for most of us). There are many theories of course, but two of the people who should know have opposite opinions: George Soros and T. Boone Pickens.
Soros says that speculation is a large part of the recently inflated cost of oil. He should know.
T. Boone Pickens says that speculation is not a very large part of the cost (5%), but rather that the current global economy demands 87 million barrels of oil per day, while the oil producers can only deliver about 85 mbpd. He should know.
They could both be correct, but in varying degrees. Or one may be wrong, and the other right.
Btw, the USA uses approx 6 billion barrels of oil per year. If we reduced our consumption about 16%, we could lower our demand by one billion barrels. That's doable. We've done it before, and grown our economy at the same time.