Welsh National Opera Roles include: Radames Aida (Summer 2008), Manrico Il trovatore (Autumn 2007), Cavaradossi Tosca (Summer 2006), Turiddu/Canio Cavalleria rusticana and Pagliacci (Summer 2005), Calaf Turandot (Autumn 2004).

Dennis O'Neill has appeared with all the world's leading opera companies. He was appointed CBE in the 2000 Millennium Honours list and awarded Verdi Medal for 2005 by Amici di Verdi.

He has enjoyed long relationships with WNO and Royal Opera where his many roles include Gustavo Un ballo in maschera, Foresto Attila, Otello, Don Carlos, Radamès Aida. At the Met he has appeared in Turandot, La bohème, Rigoletto, La traviata, Pagliacci, Aida and Cavalleria rusticana; as a regular guest at Bayerische Staatsoper in Il trovatore (c.Sinopoli), La forza del destino, Der Rosenkavalier, Tosca, Lucia di Lammermoor, Simon Boccanegra, Aida and Otello (c.Mehta).

Recent performances include Cavelleria rusticana & Pagliacci, Tosca, Turandot, Il trovatore, Il tabarro, Un ballo in maschera, Otello (Opera Australia); Pagliacci (Netherlands Opera); Otello (Hamburg, Munich, Brazil); Lucia di Lammermoor (Munich); Aida (Munich, Dresden); Un ballo in maschera (Rome); Norma (Barcelona); Pagliacci, La Juive (ROH); a tour of Verdi Requiem (La Scala); The Dream of Gerontius (Munich, London). Many recordings include solo albums, videos and operas. Future plans include Cavelleria rusticana & Pagliacci (Opera Australia) and La Juive (Netherlands Opera).