In 1905, he was appointed to the Regius Professor of Medicine at Oxford, which he held until his death. He was also a Student (fellow) of Christ Church, Oxford.
Osler is well known in the field of gerontology for the speech he gave when leaving Hopkins to become the Regius Professor of Medicine at Oxford. "The Fixed Period", given on February 22, 1905, included some controversial words about old age. Osler, who had a well-developed humorous side to his character, was in his mid-fifties when he gave the speech and in it he mentioned Anthony Trollope's The Fixed Period (1882), which envisaged a college where men retired at 67 and after being given a year to settle their affairs, would be "peacefully extinguished by chloroform". He claimed that, "the effective, moving, vitalizing work of the world is done between the ages of twenty-five and forty" and it was downhill from then on. Osler's speech was covered by the popular press which headlined their reports with "Osler recommends chloroform at sixty". The concept of mandatory euthanasia for humans after a "fixed period" (often 60 years) became a recurring theme in 20th century imaginative literature—for example, Isaac Asimov's 1950 novel Pebble in the Sky. In the 3rd edition of his Textbook, he also coined the description of pneumonia as "the friend of the aged" since it allowed elderly individuals a quick, comparatively painless death: "Taken off by it in an acute, short, not often painful illess, the old man escapes those "cold gradations of decay" so distressing to himself and his friends." Coincidentally, Osler himself died of pneumonia.