Characteristics of a good quality olive oil
In a good olive oil, a 'fruity olive aroma' stands out, i.e. a pleasant aroma that usually reminds us of the smell of fresh olive fruit, or olive leaves we rub between our fingers, but also freshly cut grass, tomato leaf, artichokes, fresh almond, vegetables, fruits, nuts or flowers as e.g. citrus.
Finally an olive oil of good quality is pleasantly 'bitter', and/or 'spicy' (hot) at the back of the mouth. The bitter and 'spicy' taste of good olive oil is due to the presence of certain desirable substances called polyphenols. Polyphenols play an important role in human health by protecting cells from oxidative stress and preventing the oxidation of bad (LDL) cholesterol. Bitter and 'spicy' olive oils are considered of higher health value.
Characteristics of a low-grade olive oil
A low-grade olive oil, has some unpleasant odors or flavors, namely 'organoleptic' defects. One of the most common defects is rancidity, which appears when an olive oil is in contact with air for a prolonged time. The 'rancid' oil has an odor reminiscent of stale walnuts or peanuts, or of old oil paint in a can.
Other common defects are 'winey' or 'vinegary' reminiscent of bad wine or vinegar, 'musty' reminiscent of mold in damp basement, 'fusty' reminiscent of rotting olives, 'cucumber', in olive oils that are stored for a long time in closed containers. There are also many other defects found more rarely ('muddy', 'sediment', 'burnt')
Source: https://www.oliveoilseminars.com/speakers/vasilis-frantzolas/?lang=en