He is a former policeman, having served with the Metropolitan Police Service and a Home Counties police force for 30 years until 1994. His experience as a policeman and of major enquiries has enabled him to find and assess evidence and "get under the skin" of certain Nazi commandants. A Russian speaker, he translated the Yiddish/Russian diary of Dimitre Gelpernus, a Jewish partisan during the occupation, thus shining a light on the destruction of the Kovno Ghetto.
O'Neil obtained his Masters degree at the University of London under Sir Martin Gilbert, another well-known writer and researcher into the Holocaust. His academic work continues with a Doctoral thesis on Belzec extermination camp, he has been researching this site for over 30 years. He has been used as a research resource or consultant for films (Schindler's List) and radio and television broadcasts. He is an honoured guest of Oskar Schindler's home town, Svitavy, Czech Republic. O'Neil has been a regular lecturer in the United Kingdom and abroad.
When asked about his investigations at Belzec, O'Neil has said: "It does not matter how big the crime is as such - it could have been 800,000 people or one person - the detection or forensic investigation, is exactly the same as long as the correct procedures are adopted.
As a counter-balance to the horrors of the Holocaust, O'Neil has researched other subjects, such as music in 19th century Vienna and Shakespeare's England. He is researching the correspondence of a Norfolk family, the Pastons, during the 15th century.