He studied privately with the renowned Frederick Cox in Manchester and London and later in his career, following a Royal Society of Arts award, in Mantua with Campogalliani and in Rome with Ricci.

He has enjoyed a long association with the Royal Opera House, Covent Garden, where his many roles have included Rodolfo (La bohème), Duca (Rigoletto), Pinkerton (Madama Butterfly), Edgardo (Lucia di Lammermoor), Macduff (Macbeth), Gustavo (Ballo in Maschera), Foresto (Attila), Otello, Don Carlos, Radames (Aida), Aroldo , Carlo (Giovanna d’Arco), Jacopo (I due Foscari) and Canio ( Pagliacci) (conducted by Domingo).

For the Metropolitan Opera he has appeared as Alfredo (La Traviata), Rodolfo (La bohème),
Radames (Aida) – conducted by Levine and Domingo - Duca (Rigoletto), Turiddu (Cavalleria Rusticana) and Canio (I Pagliacci). Other North American engagements have included Chicago Lyric Opera, San Francisco, San Diego, Vancouver Opera and concerts with the Philadelphia, Cleveland, Montreal, and Ottawa Symphonies, and for the Cincinnati Festival. He has also appeared in several South American Opera houses.

A frequent guest at the Bayerische Staatsoper, Munich, his debut in Ballo in Maschera was followed by Manrico (Il Trovatore) in a new production conducted by Sinopoli , and by Der Rosenkavalier, Tosca, Lucia, Simon Boccanegra, Aida and the title role in Otello conducted by Zubin Mehta. He has also appeared extensively for the Vienna State Opera (Boheme, Butterfly, Turandot, Traviata, Ballo in Maschera, Trovatore).

He has performed extensively throughout Europe at Opera Houses and in Festivals, visiting cities including Vienna, Berlin, Bonn, Cologne, Hamburg, Mannheim , Dresden, Hanover, Leipzig, Munich, Weimar, Halle, Dublin, Edinburgh, Paris, Nice, Marseilles, Rotterdam, Amsterdam, Antwerp, Stockholm, Oslo, Bergen, Copenhagen, Zurich, Geneva, Athens, Brussels, Antwerp, Barcelona, Madrid, San Sebastian, Oviedo, Lisbon, Porto, Milan, Rome, Bologna, Turin and the Arena di Verona.

He has retained a close and special relationship with his home based Welsh National Opera, for whom he appears in most seasons and serves as a member of the board.

Dennis O'Neill is a very busy concert artist and is well known for his performances of Verdi's Requiem having recorded it twice, broadcast many times on both television and radio and has sung with conductors including Muti, Sir Colin Davis, Sinopoli, Rattle and Pritchard. Notable concert performances have included Idomeneo with Sir Charles Mackerras at the BBC Proms, Oberto at the Edinburgh Festival, Aida in Tel Aviv with Mehta, Le Damnation de Faust in Paris with and Philadelphia with Dutoit, Tosca in Cleveland with Slatkin and The Dream of Gerontius with Svetlanov and Berlioz Te Deum with Abbado both in London. He has given recitals at Covent Garden and La Monnaie in Brussels and concerts as far afield as Sao Paolo in Brazil, Moscow, St Petersburg, and throughout Australia. He has performed in concerts before members of the Royal Families of Sweden, the United Kingdom, Norway, Spain and for the late Pontiff, John Paul the second.

Dennis O'Neill's own television series for the BBC were enormously popular (the accompanying recording went to the top of the classical record charts) and he subsequently completed a three part televison series on Caruso. His many recordings include several solo albums, videos of Der Rosenkavalier with Solti, Die Fledermaus with Kiri te Kanawa conducted by Domingo, Macbeth with Sinopoli, and Mefistofele from San Francisco. Recent releases include La fanciulla del West (Slatkin on BMG/RCA),Verdi's Requiem (Colin Davis on BMG/RCA) and with Simone Young for ABC, Tosca, La bohème, Cavalleria Rusticana, I Pagliacci, Trovatore, Aida, Turandot and two solo discs of operatic arias, all for Chandos, an anthology in three discs of Italian Songs by Bellini, Donizetti, and Verdi for Naxos. A recital of the songs of Liszt is due for release shortly and a major two year recording enterprise begins in 2006 of the complete works of Tosti. Recent operatic telecasts were of Tosca, Cavelleria Rusticana and Pagliacci for WNO and Aida from Covent Garden.

He currently visits the Sydney Opera House annually and has recently sung performances there of Cavelleria Rusticana, Pagliacci, Tosca and Il Trovatore for Opera Australia. Other recent appearances include Otello in Hamburg, Brazil and Munich, Aida in Munich and Dresden, Ballo in Lisbon, La Gioconda in concert for ENO, Turandot at the Met and Covent Garden, Ballo in Rome, Aida in Munich and Catania, Norma in Barcelona and a tour of Verdi Requiem with La Scala and Muti, and The Dream of Gerontius in Munich with Mehta.

Future commitments include a new production of I Pagliacci for Netherlands Opera, Otello in Brazil, Aida at the Teatro Colon Buenos Aires, La Juive in concert for Covent Garden, Tosca for WNO, Turandot and Trovatore at the Sydney Opera House, the Verdi Requiem in Amsterdam and Fidelio and La Juive in Brazil.

Dennis O’Neill was appointed Commander of the British Empire (CBE) in the 2000 New Year’s Honours list and many other acknowledgments most recently include the prestigious award of honorary membership of the Royal Academy of Music. (Hon.R.A.M).

He is deeply involved with the issue of education of the younger generation of Opera Singers, having given masterclasses throughout the World, specialising in the Italian repertoire and bel canto technique. He is Director of the Artists Development Programme at Wexford Festival Opera and, in association with Cardiff University, is currently forming the Cardiff International Academy of Voice of which he will be director from its opening in 2007. He serves on numerous panels of adjudicators at various international competitions