He received a general education at the Kingsbury School in Middlesex before serving an apprenticeship with the London printers William Clowes &amp; Sons Ltd. Osborne joined the Territorial Army at the age of twenty serving as a lorry driver for the Royal Artillery in France during World War Two. Gunner Osborne had to be evacuated from the beaches of Dunkirk in 1940. He almost drowned as he waded out to meet his rescuers. He was later re-deployed with the First Armoured Division in North Africa in 1941. In January 1942 Dennis was taken prisoner of war and transported to a prison camp in the Dolomites of northern Italy. Dennis and a friend escaped when the guards abandoned their posts after Italy's capitulation in July 1943. They spent six months on-the-run before being recaptured and transferred to a forced labour camp in Silesia, Poland, where they remained until their liberation by the American military in 1945.