Two analytical methods, based on the difference between theoretical and empirical triglycerides and the ratios between some free and esterified sterols have been checked to determine their usefulness in detecting the presence of low quantities of any kind of hazelnut oil in olive oil. The methods were confirmed by means of internal and external validations, the latter carried out in 21 different laboratories in three inter-comparison trials. The resulting information suggests a cut-off at 8% for the method based on triglycerides and 10% for that based on the quantification of sterols. The former also shows better reliability measures in reproducibility; i.e., number of false positives, efficiency (90% vs. 82%) and Youden index (0.81 vs. 0.77).