Onomarchus (Ancient Greek: Ὀνόμαρχος) was general of the Phocians in the Third Sacred War, brother of Philomelus and son of Theotimus.
As far as the funding of his campaign was concerned, Onomarchus went a step further than his brother and predecessor, who had 'borrowed' from the sacred treasures of Delphi, keeping, however, scrupulous records.
Using the treasures of Delphi he was able to assemble and maintain a large body of mercenary troops, in addition to bribing many of the hostile states, allowing him to influence the Thessalians to abandon their allies and take up a neutral position.
Onomarchus then marched with his entire force to support Lycophron, defeated Philip in two successive battles, and drove him out of Thessaly.
[6] Reportedly, Onomarchus and many of the fugitives plunged into the sea in the hope of swimming to the Athenian ships under Chares which were lying off the shore.
Some sources relate that he also killed 3000 Phocian hostages as a punishment for the sacrilege they committed against the Sanctuary of Apollo at Delphi two years earlier.