The ancient world is rife with stories of people chatting with gods and predicting the future (and having the prophesy come true). However, whereas you would dismiss such claims from another religion, you accept them in yours.
Arguments over the correct interpretation of an oracle were common, but the oracle was always happy to give another prophecy if more gold was provided. A good example is the famous incident before the Battle of Salamis when the Pythia first predicted doom and later predicted that a 'wooden wall' (interpreted by the Athenians to mean their ships) would save them.
That kills her success rate right there.
You mentioned "writers" with regard to the oracle. The Bible is a compilation of 66 books written by over 40 different authors, some of them prophets, some of them not, but all commonly linked by their belief in YHWH, God of Israel, and their remarkably consistent message.
What is the Greek counterpart to that which lends support to the existence of the Greek gods, preserves their message and details the prophecies of those such as the oracle?