Ong was an ethnic Chinese born in Kuching, Raj of Sarawak. He went to boarding school in Singapore, where his natural manual dexterity and his determination to achieve perfection first became manifest.  Appointed as school barber but with no practical experience in the art, he trained himself to the high standards that were to become his hallmark by paying his fellow scholars for the privilege of cutting their hair. Having decided on a career in medicine, his application to Singapore Medical College in April 1940 was turned down for 'logistical reasons'. He entered the University of Hong Kong to study medicine, but in 1941 when the Japanese invaded Hong Kong, the University was determined to continue its activities in mainland China. As a result, after a hazardous trek on foot lasting some three months, Ong and some fellow students were able to resume studies in Chongqing. Ong subsequently graduated with an MD from the Shanghai Medical School.   After the war he graduated with a MBBS from the University of Hong Kong in 1947.