He received critical acclaim and won a Laurence Olivier Award for playing Michael "Mickey" Johnstone in the musical Blood Brothers.

[citation needed] Aged one, O'Neill moved with his family to Parbold, growing up in the village of Up Holland, both near Wigan, in Lancashire.

He appeared as wheelchair user PC Ian LeFebre in "The Mild Bunch", the second season's eighth episode of Pie in the Sky (1995).

In 2008, he appeared in Criminal Justice, a five-part TV drama on the BBC, playing the part of Ralph Stone, a solicitor.

In 2011, he took the role of dock worker Eddie Carbone in Arthur Miller's A View From the Bridge at the Royal Exchange Theatre in Manchester (from 18 May to 25 June 2011).

In 2016, O'Neill played the part of Joe Brierley in the second series of Ordinary Lies and appeared in two episodes of the Doctor Who spin-off Class.

O'Neill portrays Gotham City Police Department's Chief Mackenzie Bock in minor roles for both the feature film, The Batman (2022) and its follow-up TV miniseries, The Penguin (2024).