O’Neill studied with Sir Arthur Somervell from the age of fourteen, until, on the suggestion of the celebrated violinist Joseph Joachim, he went to study composition at Frankfurt with Professor Ivan Knorr. There, O’Neill found several other immensely talented young composers – Roger Quilter, Balfour Gardiner, Percy Grainger and Cyril Scott. The five students learnt a great deal from each other, and were later referred to as the ‘Frankfurt Gang’. Much of O’Neill’s subsequent working life was spent in the theatre, and he was Musical Director of the Haymarket Theatre for many years. As well as conducting, he composed incidental music, producing more than fifty scores, including the highly acclaimed music for J.M. Barrie’s play Mary Rose.

He was also well respected as a teacher and he held a number of posts, primarily at the Royal Academy of Music. He was also associated with St Paul’s Girls’ School in Hammersmith and took over the school orchestra from Gustav Holst when the latter took leave to carry out educational work in Salonica during the First World War.

O’Neill’s output included a few ballets, and a wide range of chamber, choral, orchestral and instrumental music. He produced well-crafted pieces, full of charm and an easy delight; works which reflected both the English countryside he so deeply loved, and his own warm, charming and sensitive personality.