Quality & taste

The factors that makes one oil taste so different from another:

Region
Influences that differentiate olives from region to region are soil. weather, climate and vintage. Each will leave its trace in the final product.

Handling
As with any fruit, bruising is a problem that causes rapid deterioration. Careful hand picking will ensure that the bruising is minimal, and that only the olives at the proper stage of ripeness will be picked.

Season
If an olive oil is made from olives picked early in the season, it will have a fruitier (more olive-like) and less sweet flavor. This oil is generally more costly because early harvest olives have a lower oil content.

Time
An olive is at its best the moment it is picked. Ideally, it should be delivered to the press within one day else natural deterioration will begin to affect the flavor, complexity and character of the oil. For this reason, olive growers are usually served by presses in their local area.

Varietal
There are many varieties of olives grown for oil. Like grapes for wine, each type of olive will contribute its own characteristics to the oil pressed from it.

Taken from: A Guide to Good Olive Oil , by Ari Weinzweig, Zingerman's and Dancing Sandwich Enterprises.