Oscar Diethelm was born at Manly, NSW, the second son of Albert Diethelm, a dentist from Lachen, Switzerland, and Rosa Angela (nee Ghiggino) from Genoa, Italy. His parents died when he was quite young leaving him to fend for himself. This he did most successfully. From the age of eleven a bursary enabled him to complete his secondary education at Sydney Boys’ High School, finishing in 1900 as school captain and dux. A bagful of prizes, scholarships and exhibitions at the matriculation examinations, and in first year arts financed him through two years of arts and five years of medicine at the University of Sydney, while a resident at St John's College. He graduated in 1907 with honours.

After being a resident medical officer at Royal Prince Alfred Hospital in 1907, he moved to Grafton, NSW to partner in general practice Dr JT Henry, who retired in 1921, his replacement being Dr Eric Preston Holland. Oscar's brilliant academic career was surpassed by his career in practice, and his civic, hospital and ambulance activities. He was the beloved physician and citizen. When he left Grafton after fourteen years, the only people happy to see him depart were the bookmakers and his poker opponents.

In 1922 he sailed overseas to undertake postgraduate studies at St Thomas’ Hospital in London, and visited many of the continental clinics. Highlights of this venture were his attendance at the BMA conference at Portsmouth where Banting delivered his paper on the discovery and use of insulin, and the acquisition of a stethoscope with combined bell and diaphragm which he used for all his teaching days, although I never saw him use anything but the bell.

When St Vincent's Hospital became a clinical school in 1923, the Hospital authorities showed great courage and discrimination in appointing Oscar as senior physician and clinical lecturer. He worked indefatigably and enthusiastically for the Hospital until he died. He set up as consultant physician at Craignish, Macquarie Street, where he continued until he retired. The practice was huge and lucrative, he was soon known by many people, rich and poor, loved by many and respected by all. Other appointments were fellow of St John's College, councillor of The Royal Australasian College of Physicians, member of the advisory board of St Vincent's Hospital, director of Prince Henry Hospital, and consultant physician for HM Government in Fiji and the Western Pacific High Commission.

This extraordinarily busy and successful life would not have been possible without the support and encouragement of his wife Isobel Mary (nee Coleman) BA, whom he married on 6 July 1910, with Herbert Schlink the best man. She also provided four sons of whom only Oscar did not do medicine. Anton (Tony), Basil and Geoffrey graduated in medicine from the University of Sydney. It was a very happy marriage and a fine family unit. Basil's death at the age of twenty-nine in 1946 was a bitter blow to the family. That was the year of Oscar's retirement from St Vincent's teaching staff. He wife predeceased him by a few months in 1962.

Of the hundreds of us who were privileged to be his students, none would remember him without admiration, gratitude and respect. He was tidy in dress and in habits, meticulously punctual and always well prepared for lectures and ward rounds. He had infectious enthusiasm, was serious but jolly, was always in a hurry, never having enough time to impart all the wisdom, stored in a most capacious brain that forced his words to tumble over one another, so that `costal cartridges’ would not be misinterpreted by us, nor the necessity for `large doses of PR’ for the man with melaena who must have full investigation and treatment with iron. Even when he enthused on a classical papilloedema with the ophthalmoscope light shining in his own eye, we did not lose love or respect. He made us feel we were his best students and he was able to make the patients feel the same of themselves.

He loved work, loved his wife and family, loved his students, patients and friends, loved his religion and life itself, but when he was to keep his appointment with death he was thoroughly prepared and was not late. We lost an unforgettable character.