Not all of the olive trees around town are decorative. Some people process olives for personal consumption in crocks at home and others process the olives for oil commercially. There is also the UC Davis Olive Center to direct all your olive questions to.
UC Davis has even been the host for international olive oil conferences.
The various grades of Olive Oil are explained here in a discussion of grade inflation.
In a 2010 New York Times article questioning the suitability of olive oil in cooking, they repeatedly cite UC Davis studies and also use a panel of tasters from an "olive oil research group at Davis" to compare oils.
According to Selina Wang at the Olive Center, most olive oils imported from Italy—or anywhere outside the U.S.—have already been waiting a year and by the time they hit grocery mart shelves, have expired. It's a good reason to buy California olive oil—get actual fresh, high-quality oil, and support a local businesses! Learn more here http://olivecenter.ucdavis.edu/contact-us/publications (Davis Olive Center Resource page)