Onomacritus (Greek: Ὀνομάκριτος; c. 530 – c. 480 BCE), also known as Onomacritos or Onomakritos, was a Greek , or compiler of oracles, who lived at the court of the tyrant Pisistratus in Athens. He is said to have prepared an edition of the Homeric poems, and was an industrious collector, as well as a forger of old oracles and poems.
Herodotus reports that Onomacritus was hired by Pisistratus to compile the oracles of Musaeus, but that Onomacritus inserted forgeries of his own that were detected by Lasus of Hermione. As a result, Onomacritus was banished from Athens by Pisistratus' son Hipparchus. After the flight of the Pisistratids to Persia, Onomacritus was reconciled with them. According to Herodotus, Onomacritus induced Xerxes I, the King of Persia, by his oracular responses, to decide upon his war with Greece.
Pausanias attributes to Onomacritus certain poems forged under the name of Musaeus. In explaining the presence of the Titan Anytos at Lycosura, he says that "From Homer the name of the Titans was taken by Onomakritos, who in the orgies he composed for Dionysos made the Titans the authors of the god's sufferings." Therefore, Onomacritus is responsible for inventing an important aspect of the mythology concerning the Titans.