[1] He appeared for Joanna Cherry MP before the Supreme Court of the United Kingdom in the September 2019 case relating to the prorogation of Parliament.
[3][1][4] From 1997 to 1999 O'Neill was Standing Junior Counsel to the Scottish Office Education and Industry Department.
[1] He was awarded a doctoral degree (LL.D) from the University of Edinburgh in 2012 on the basis of his work's academic merits, independent scholarship and legal substance of published books.
[6] He was appointed by a cross-party group of 75 MPs and peers to contest a case for judicial review against the Government in the Scottish Court of Session.
He went on to receive widespread media coverage in September 2019 upon representing the group including Joanna Cherry MP against the government (represented by Advocate General for Scotland, Lord Keen QC) in the case of R (Miller) v Prime Minister; Cherry & Ors v Advocate General for Scotland.
[2] He was cited in media reports as passionately arguing to the justices that the "Mother of parliaments been shut down by the father of lies".