The Oracle of Delphi always had to tell the truth, but she could also never say yes or no to anyone. The oracle always just had to make an honest statement, and it was up to the listener to determine what it meant. Often, people would misunderstand what she was telling them. That was the trouble with oracles, even the best ones like Pythia.
Pythia had to be an older woman who had a blameless life before becoming the oracle. She would be chosen by the peasants of the area.