2007 article in The New Yorker by Tom Mueller: "Olive-oil fraud was already common in antiquity." But as consumption of the oil has increased ("thirty-five per cent in southern Europe, its traditional market, and more than a hundred per cent in North America" over the past decade), adulteration, marketing scandals, and other forms of false advertising have risen as well. The website Truth in Olive Oil contains some helpful information to guide the consumer, but in general you should look for a deep green oil, bottled in dark glass, certified organic, and of course "extra-virgin" (although this term is often applied to a less than virginal product). Your best bet for where to place your money would seem to be, if counter-intuitively, California. One of my favorite insights from Mueller is that real olive oil is "fruit juice." Not-so-real olive oil is an inferior mix of improperly prepared olives mixed with other types of oil. OhioLINK reveals six impurities with regard to today's typo; there were 63 found in WorldCat.
(Italian olive oil, from the city of Manerba del Garda, northern Italy, July 2007, from Wikimedia Commons.)