Olives reach their full size in the fall but may not fully ripen from green to black until late winter. 
Green olives have slightly less oil, more bitterness and can be higher in polyphenols. 
The oil tends to be more expensive because it takes more olives to make a bottle of oil. 
Many people like the peppery and bitter quality of early harvest oil. 
Flavor notes of grass, green, green leaf, pungent, astringent are used to describe early harvest fall oils. 
Because of the higher polyphenols and antioxidants, early harvest oils often have a longer shelf life and are blended with late harvest oils to improve their shelf life.

( Noah Gourmet Early Harvest Extra Virgin Olive Oil, which has low acidity levels 0.6) .