Green’s Dictionary of Slang

(Irish) a street challenge, esp. from a weaker to a stronger person or group.

1939		‘Flann O’Brien’ At Swim-Two-Birds 165: Don’t make any mistakes about that if you value your bloody life. H.O., H.A. Hit one, hit all.
1946		D. MacDonagh Happy as Larry Act IV: H.O.H.A., Hit one, hit all, / Here’s when the wake becomes a spree.
1963		L. Daiken Out Goes She 12: H-O-H-A [...] The initial letters for Hit One Hit All! a defensive challenge when a tough gang is menaced by a tougher gang.
1981		(con. c.1920) P. Crosbie Your Dinner’s Poured Out! 55: Meeting a rival gang, members of each warned the other with H.O.H.A. which meant ‘hit one, hit all’.