In July 1931, Bretaigne Windust, Charles Leatherbee (the grandson of Charles Richard Crane), and Joshua Logan, the three directors of the University Players, a three-year-old summer stock company at West Falmouth on Cape Cod, were looking for a leading lady for their repertory season that winter in Baltimore.

At the suggestion of George Pierce Baker, they auditioned and hired O'Neil, one of his talented students at the Yale School of Drama.

Her other stage credits include originating the role of Madam Serena Merle in a Broadway adaptation of The Portrait of a Lady in 1954.

[7] The following year, she appeared in All This, and Heaven Too (as the wife of Charles Boyer); she was nominated for the Academy Award for Best Supporting Actress for the role of the domineering and jealous Duchesse de Praslin.

O'Neil died from a heart attack at the age of 70 on September 3, 1980[9] at her home in Cos Cob, Connecticut.