After a career as the British major crimes' investigator who worked on criminal investigations for Scotland Yard, the Metropolitan Police Service, and London Home Counties Police, he obtained his Masters and Doctorate with the Hebrew and Jewish Department at University College London.
He has launched a number of investigations into the perpetrators of the Holocaust, particularly those active in Lithuania and occupied Poland during World War II.
He has conducted research regarding the Schutzstaffel (SS) and extermination camp commandants of Belzec, Sobibor, and Treblinka.
A historical consultant to several TV documentaries and radio broadcasts in the UK and abroad, he is an honoured guest of Schindler's home town, Svitavy, Czech Republic, and is a regular lecturer at universities in the United Kingdom, United States, Israel, and Eastern Europe.
O'Neil reveals how the euphemistic language spoken within the Nazi State allowed for the sanitization of genocide and the creation of the complete illusion of 'plain speak' in phrases such as 'treatment', 'processing', and 'resettlement' which enabled the SS to turn mass murder into a "bureaucratic paper chase".