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.

[3] In 2004, O'Neill was the subject of a TVNZ, BBC documentary The Understudy[4] about his contract at the Metropolitan Opera to cover Siegmund for Plácido Domingo in the Met's Otto Schenk production of Die Walküre.

O'Neill contributed as tenor soloist to the Deutsche Grammophon recording of Mahler's "Symphony No.8" with the Los Angeles Philharmonic conducted by Gustavo Dudamel.

The recording received two nominations for the 2022 Grammy Awards for Best Choral Performance and Best Engineered Album in Classical.

In the 2017 Queen's Birthday Honours, O'Neill was appointed an Officer of the New Zealand Order of Merit for services to opera.