O'Neil's first performance in a professional production was in the role of a nun in "Sarah" at the Alcazar Theatre in San Francisco on October 16, 1893. Before returning to San Francisco in 1898 and 1899 as a star, headlining in the plays "The Jewess" and "The Shadow", she spent the preceding years honing her acting skills by playing in every type of venue, "from barns to first-class theatres", in towns throughout the country's West and Northwest. O'Neil later described that early period of her career as a time when she appeared "in fully a hundred characters, varying from soubrettes to heavies."