The most critical variable (and least understood!) in producing fine olive oil is the ripeness level of the olive fruit when they are harvested. The level of ripeness affects both yield and organoleptic characteristics. Additional factors that affect the quality of a top notch EVOO include harvest time, variety of olive, weather like frost or deluges, cleanliness of equipment, and mill schedules. Once the olives are picked, time is of the essence. Every moment the olive sits after it leaves the tree, it degrades. The faster the olive is pressed, the more likely the final product will be of high quality.
– Paul Vossen