O'Neill was born in Ashburton, New Zealand and received his musical training at the University of Otago, Victoria University of
Wellington, graduating with an honours degree in music, before receiving scholarships to the Manhattan School of Music (where he
earned a Master of Music degree in 2000) and the Juilliard Opera Center.
While studying at the Juilliard Opera Center, he sang the title role in Idomeneo, Sam Polk in Susannah, and Chevalier de la Force
in Dialogues of the Carmelites under the baton of Julius Rudel. Subsequently at San Francisco's Merola Opera Program, he performed
Rodolfo as well as the title role in La clemenza di Tito with Wolf Trap Opera. In 2004, O'Neill was the subject of a
BBC documentary The Understudy about his contract at the Metropolitan Opera to cover Siegmund for Plácido Domingo in the MET's
production of Die Walküre.
He made his MET debut as the Gran Sacerdote in Idomeneo with James Levine, his Covent Garden debut as Jenik
in Prodaná nevesta with Charles Mackerras followed by Florestan with Antonio Pappano and Salzburg Festival
in Die Zauberflöte with Riccardo Muti. O'Neill toured Japan with the MET covering Plácido Domingo in the role of Siegmund.
Other roles include Mitch in Previn's A Streetcar Named Desire at Theater an der Wien, Rodolfo in New
Orleans, Grand Rapids and San Francisco, Don José and Tito (La clemenza di Tito) at Wolf Trap, Chairman Mao in John Adams
' Nixon in China with Minnesota Opera, Dimitri with New Zealand Opera, Skuratov in From the House of
the Dead in New York, Canio in concert with the Auckland Philharmonia, Sam in Floyd's Susannah at the Wexford Festival,
Chevalier de la Force (Dialogues of the Carmelites) at Juilliard and recorded the role of Lancelot in Chausson's Le roi Arthus
with the BBC Symphony Orchestra.
At extremely short notice he made his 2009 debut in the title role of Otello with Colin Davis and the London Symphony
Orchestra and in 2010 he made his debut at the Bayreuth Festival as Lohengrin. In 2009 he performed
Lohengrin at Covent Garden and Houston Grand Opera, Sergei in his Opera Australia debut in Lady Macbeth of Mtsensk and
Florestan with Daniel Barenboim at the BBC Proms in Fidelio. Other performance highlights have included Parsifal at the Accademia
Nazionale di Santa Cecilia in Rome conducted by Daniele Gatti, debuts as Siegmund in Die Walküre at Covent
Garden, with Antonio Pappano and at the MET with Donald Runnicles, toured Europe with the West-Eastern Divan
Orchestra and Daniel Barenboim and made his Carnegie Hall debut as Caesar in Samuel Barber's Antony and Cleopatra.