An oracle can be a person, a place , or a thing. As a person an oracle is a priest or priestess who administers a shrine. Such oracles were considered to be in close contact with a god and thereby able to act as mediums to the god. In this fashion, Greeks could pilgrimage to the oracle in order to ask important questions of the gods and receive their answers. Until late in the history of Delphi, the medium was always a young woman. She would sit on a tripod, go into a state of frenzy, and then relay to the person who asked the question the response of the god. It is worthy of note, that despite the male dominated world of the ancient Greeks this role of direct communion with the god Apollo at Delphi was reserved for a woman. Least we go too far in interpreting the significance of this, however, note that the god's answers were often in riddle form, amphibolies, requiring an interpretation which was done by the shrine's priest. Thus, the old stereotype that women are quite emotional while men are very rational has been around at least since ancient times.
As a place, an oracle was the holy site itself. There were many such shrines in classical times with the greatest being at Delphi where Apollo, the most esteemed of the oracular gods, resided.
Finally, as a thing, it was the pronouncement
of the god itself. This is the primary signficance of the word.