Oracle at Delphi
The Delphic oracle was the most important oracle in the classical Greek world. The priestesses of the oracle were known as the Pythias. It was believed to be the most truthful one.
Delphi (Greek: Δελφοί) is at present an archaeological site and a modern city in Greece. It is located on the south-western spur of Mount Parnassus in the valley of Phocis. The god Apollo had a temple in this place; it was its major site of worship.
The prophecies of the Oracle were notoriously ambiguous. There are many examples.
As the massive hordes of the Persian army approached Athens, the town leaders asked the Oracle what the best way to repel the Persian attack was. The Oracle told them to "trust in wooden walls," but a debate ensued among the leaders of Athens whether this meant to build large wooden walls around the city, or to use their navy (the hulls being "wooden walls") to repel the invaders. Eventually, it was decided the naval interpretation was the correct one, and the Athenians won a spectacular naval encounter at the Battle of Salamis. This does not validate the Oracle - if they had lost the battle, it would have been assumed that they should have built wooden walls around the city.
When King Croesus wanted to know if he should invade Persia, the prophecy was that "If Croesus makes war on the Persians, he will destroy a great empire." Indeed, he did - his own. Note again that this prophecy was worded in such a way that it could not possibly be proven wrong - no matter who won, it would have been true.