On graduating from Guildhall, O'Neil was awarded the 2009 Spotlight Showcase prize for Best Actress.
In July 2011 she became the face of the Match.com advertising campaign, in "The Girl On The Platform". O'Neil and her Match.com advert co-star, Steven Mitchell, also appear in the music video for 'She Began To Dance' sung by Matthew P. The music video shows the couple in similar but not identical outfits leaving the unidentified train station and going on a date to the beach. This suggests the advert and the music video were not filmed at the same time. Matthew P plays most of the 'characters' in the video whilst simultaneously singing his song with a ukulele.
In 2012 O'Neil appeared in two premieres at the National Theatre: in Nicholas Wright's Travelling Light and James Graham's This House.
In 2014 O'Neil appeared in King Lear alongside Frank Langella and in Richard III with Martin Freeman. In 2015 she appeared in a 2-part episode "Squaring the Circle" in Series 18 of BBC One's Silent Witness as young City DI Sarah Parks. She played Rowena in the Midsomer Murders season 18 finale 'Harvest of Souls'.
