While OPEC is a cartel, it does not fix prices. It reduces or increases supply, and then the market decides price organically based on laws of supply & demand. So not quite the same.
That's like saying the sky isn't really blue, when someone asks you the color. "As far as wavelengths go, Earth's sky really is a bluish violet. But because of our eyes we see it as pale blue." [1]
Saying OPEC doesn't fix prices is a nonsense statement that does nothing but split hairs. OPEC engages in price fixing as set forth by the FTC, though they are not subject to the FTC. [2]
Price fixing is a term of art that doesn't just mean the specific price of a sale is fixed. Price fixing can also mean that prices are indirectly fixed by production quotas or capacity. OPEC exists for the very reason to set production quotas or capacity. "OPEC's objective is to co-ordinate and unify petroleum policies among Member Countries" [3]
The key to price fixing is being an entity larger than the government can control and having something the government desperately needs. It also helps when is a globally desirable product and the ability to ship it anywhere.
Still, you can find more examples of OPEC members going against the wishes of OPEC than you can find landlords going against the wishes of RealPage. Good thinking though!
Uh, OPEC?