O'Neill attended the Guildhall School of Music and Drama, which he graduated in 2009. At the end of the Guildhall, O'Neal was awarded the 2009 Best Actress Award. In July 2011, she became the face of Match.com advertising campaign, in the movie "Girl on the Platform." In 2014, O'Neill appeared in "King Lear" with Frank Langella and in "Richard III" with Martin Freeman.