O'Neil attended Lester B. Pearson High School[5] in Calgary, Alberta, where she performed in musicals, played rugby and was on the basketball team.

After scoring the highest number of votes in her semi-final group, again with "Concrete Angel", O'Neil was granted a place in the final 10 of the contest.

From then on, O'Neil never fell back into the bottom three after delivering consistently praised performances, with judge Zack Werner declaring that she was "a threat to win the whole thing".

On September 7, 2005, O'Neil made it into the final two of the competition, alongside Rex Goudie, and after more performances, judge Werner stated that the headlines of the next day's newspapers would be "The King [Kalan Porter] is dead, long live the Queen."

In March 2006, her debut album was certified gold by the CRIA for 50,000 units shipped, and her first single "Alive" was covered by Becki Ryan in the movie Flicka.

[13] In the Drayton Entertainment production of High School Musical On Stage (in St. Jacobs and Penetanguishene), O'Neil played the role of Gabriella.

[21] In March 2012, O'Neil made her Broadway debut in Jesus Christ Superstar, where she played Martha, Maid by the Fire and worked as understudy to the role of Mary Magdalene.

[23][24] In March 2014, O'Neil returned to Broadway in a revival of Les Misérables, where she was a member of the ensemble and worked as understudy for the roles of Éponine and Fantine.

[27] In December 2014, O'Neil was cast in the starring role of Two / Rebecca / Portia Lin, on the Syfy TV series space opera Dark Matter.