Olive oil is the only commercially significant vegetable oil sold retail to be extracted from a fruit rather than from seeds. Since the fruit contains considerable water, extraction can be done by mechanical methods alone, with a centrifuge or press, whereas extracting seed oils requires the use of industrial solvents, typically hexane.
To remove this solvent from seed oils, as well as to eliminate the unpleasant tastes and odours they normally have, they must be processed in a refinery, where they undergo high-temperature desolvenization, neutralization, deodorization, bleaching and degumming. The end result is a tasteless, odourless, colourless, liquid fat.
Olive oil, instead, can simply be pressed or spun out of the olive pulp, yielding a fresh-squeezed fruit juice with all of its natural tastes, aromas and health-enhancing ingredients intact.
Extracted from Extra Virginity - the Sublime and Scandalous World of Olive Oil by Tom Mueller, available here.