An estimated 69% of all store-bought extra virgin olive oils in the US are probably fake, according to tests by the University of California. UC Davis tested samples from the top-selling extra virgin olive oil brands to find the ones that are not worth buying and those that are.
The article lists the offenders as well as those who passed the testing.
For a run down of the corruption and mafia involvement, check this New Yorker article: Slipper Business
In 1997 and 1998, olive oil was the most adulterated agricultural product in the European Union, prompting the E.U.’s anti-fraud office to establish an olive-oil task force. (“Profits were comparable to cocaine trafficking, with none of the risks,” one investigator told me.) The E.U. also began phasing out subsidies for olive-oil producers and bottlers, in an effort to reduce crime, and after a few years it disbanded the task force. Yet fraud remains a major international problem: olive oil is far more valuable than most other vegetable oils, but it is costly and time-consuming to produce—and surprisingly easy to doctor.