O'Connor was born in Worcester, Massachusetts and died in Dublin, Ireland. For a time he was in the London studio of the painter, John Singer Sargent, and later worked for the architects, McKim, Mead and White in America and with the sculptor Daniel Chester French. Settling in Paris in the early years of the 20th century, he exhibited annually at the Paris Salon. In 1906 he was the first foreign sculptor to win the Second Class medal for his statue of General Henry Ware Lawton, now in Garfield Park in Indianapolis. In 1928 he achieved a similar distinction by being awarded the Gold Medal for his Tristan and Iseult, a marble group now in the Brooklyn Museum.
A number of his plaster casts are in the Hugh Lane Municipal Gallery, Dublin and there are works in Tate Britain, the Walters Art Museum, Baltimore, the Corcoran Gallery of Art, the Metropolitan Museum of Art and the
Musée d'Art Moderne, Paris.
O'Connor was involved in a minor controversy in 1909 when he was commissioned to design a statue for Commodore John Barry, of the American Revolutionary era navy. O'Connor's first design was heatedly attacked by Irish-American groups; he submitted a second version, but it too was ultimately rejected and John J. Boyle received the commission.