Sir Con Douglas Walter O'Neill GCMG (3 June 1912 – 11 January 1988) was a British civil servant and diplomat.

He gained a fellowship at All Souls College, Oxford, in 1935 before joining the Diplomatic Service in 1936.

In 1939, with the outbreak of the Second World War, he entered the Army Intelligence Corps.

He joined the German Section of the Political Warfare Executive, during which time he interrogated Adolf Hitler's former deputy, Rudolf Hess.

His daughter Onora is now Baroness O'Neill of Bengarve.