A new method for direct measurement of the oleocanthal and oleacein levels in olive oil by quantitative (1)H NMR was developed. The method was applied to the study of 175 monovarietal commercial Greek and California olive oil samples. The main findings were as follows: (1) There was a significant variation concerning the concentrations of oleocanthal and oleacein among the studied samples. Their concentrations ranged from nondetectable to 355 mg/kg and their sum (index D1) from 0 to 501 mg/kg. (2) There are olive varieties that independent of geographic origin and harvest time produce oil that contains both compounds in low levels. (3) There is a positive correlation of a high level of oleocanthal and oleacein in olive oils with the early time of harvest. Although there is a need for more extensive study, a new index for the characterization of extra virgin olive oils, which is a combination of D1 = oleocanthal + oleacein level and D2 = oleocanthal/oleacein ratio, seems to be very useful.