The word 'oracle' can mean either a divine prophecy, or the place where these prophecies were given. In these shrines where oracles were given , priests would interpret for the petitioner the word of the gods through a sign.
Each oracle would have its own way of consulting it particular god. The two most famous oracles of the ancient world were those of Zeus in Epirus and Apollo at Delphi, the ruins of which are pictured on the left.
At the Oracle of Zeus in Epirus, priests apparently interpreted the rustling of the wind through a sacred oak tree. These priests were called Selloi, and were 'of unwashed feet and sleeping on the ground.'
At Delphi, located on the side of Mt. Parnassus where the stone Cronus mistook for Zeus landed, a priestess over the age of 50, called a Pythia, was placed upon a tripod. Later writers state that this tripod was located over a fissure, from which fumes would rise. After she was asked a questions, the Pythia would then achieve an ecstatic state and produce the oracle, which must have been chaotic at best. The priests would then interpret these ramblings for the petitioners, often into poetic meter.
Those seeking an oracle would have left an offering, making the more popular oracles and the priests who attended them quite wealthy. Bribery for a more favorable oracle certainly was not out of the question.
Oracles, especially the Oracle at Delphi, had a tremendously powerful impact upon the ancient Greek world. Powerful men, cities and nations would consult them before making any major decision. Often times, oracles appear to have taken on their own political agenda. In the Persian Wars, the Oracle at Delphi clearly favored the Persians over the Greeks. In the Peloponnesian Wars, it favored the Spartans over the Athenians.
Oracles retained their hold upon Greek society until the first century AD. At that time, they seem to have been replaced by astronomy, a medium much more accessible than the oracles.
Plutarch. The Fall of the Roman Republic.
Burkert, Walter. Ancient Mystery Cults. Harvard Press, 1987.
Herodotus. The Persian Wars.
Thucydides. The Peloponnesian Wars.
Oxford Concise Companion to Classical Literature. ed. Howatson and Chilvers. Oxford. New York, 1993.