Onomarchus commanded a division of the Phocian army under Philomelus in the action at Tithorea, in which Philomelus perished. After the battle Onomarchus gathered the remains of the Phocian army and retreated to Delphi. An assembly of the people was held, in which Onomarchus strongly urged the prosecution of the war — in opposition to the counsels of the more moderate party.
In the winter of 354 / 353 the Phocians decided to make Onomarchus supreme command,  in place of Philomelus. He was not inclined to imitate the moderation of his predecessor, who had 'borrowed' from the sacred treasures of Delphi and had kept scrupulous records. Instead, he confiscated the property of all those states who were opposed to Phocis and made full use of the accumulated wealth of the shrine. 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.Thus freed from his most formidable antagonists, he was more than a match for his remaining foes.
Onomarchus' campaigning began in 353. 
He invaded Locris, took the town of Thronium, compelled Amphissa to submit, ravaged the Dorian Tetrapolis, and finally turned his arms against Boeotia — where he took Orchomenus and laid siege to Chaeronea — but was compelled to retreat without effecting anything more.
Following the siege of Chaeronea, his assistance was requested by Lycophron, a tyrant of Pherae who was attacked by Philip II of Macedon. At first Onomarchus sent his brother Phayllus into Thessaly with an army of 7000 men, to a defeat by Philip's armies. Onomarchus then marched with his whole forces to the support of Lycophron, defeated Philip in two successive battles, and drove him out of Thessaly.
Onomarchus then returned to Boeotia, whom he defeated in a battle, and took the city of Coroneia. He was then recalled once more to the assistance of Lycophron, against Philip, who had again invaded Thessaly. Onomarchus hastened to support his ally with an army of 20,000 foot and 500 horse (another source shows 3000 horses ), but was met by Philip at the head of a force, still more numerous, and a pitched battle ensued, in which the superiority of the Thessalian cavalry decided the victory in favour of Philip.
There are differences in the details provided from those sources which provide accounts on the events involving his death. 
Onomarchus and many of the fugitives plunged into the sea in hopes of swimming to the Athenian ships under Chares which were lying off the shore.
His death took place in 352 BC.
Onomarchus' body fell into the hands of Philip, who had it crucified as punishment for his sacrilege.
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- Charles Anthon - A Classical Dictionary: Containing ... Proper Names Mentioned in Ancient Authors, and Intended to Elucidate ... Points Connected with the Geography, History, Biography, Mythology and Fine Arts of the Greeks and Romans ... an Account of Coins, Weights and Measures Harper & Bros., 1841 [Retrieved 2015-0412]
This article incorporates text from a publication now in the public domain: Smith, William, ed. (1870). "article name needed". Dictionary of Greek and Roman Biography and Mythology.
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