Sir Oswald Hornby Joseph Birley MC RA ROI was an English portrait painter and royal portraitist in the early part of the 20th century.

Birley was born in New Zealand to Hugh Francis Birley (1855–1916) while his parents were on a world tour. He was born into an old Lancashire family. Upon returning to England, he was educated at Harrow School, London and Trinity College, Cambridge.

He was the great-grandson of Hugh Hornby Birley (1778–1845), who led the troops at the Peterloo massacre.

Birley served in France in World War I, first with the Royal Fusiliers, where in April 1915 he gained the temporary rank of lieutenant, later transferring to the Intelligence Corps, obtaining the rank of captain and being awarded the Military Cross in 1918.

During World War II he served with the rank of major in the Home Guard.

A favourite of the Royal Family, Birley was well known for his portraits of King George V, Queen Mary, King George VI, Queen Elizabeth, the Queen Mother and Queen Elizabeth II.

He painted several highly regarded portraits of Sir Winston Churchill (to whom he also gave lessons), and also a life-size portrait of Mahatma Gandhi which was the first to be hung in the Lok Sabha shortly after Indian Independence on 28 August 1947.

Other subjects were many war-time leaders such as Generals Eisenhower and Montgomery, as well as Admiral Mountbatten and Air Marshal Trenchard. He also painted the wealthy American financiers Andrew Mellon and J. P. Morgan, the psychiatrist Sir James Crichton-Browne, Welsh architect Sir Clough Williams-Ellis and art historian Sir Robert Witt, co-founder of the Courtauld Institute of Art. Birley painted the portrait of Leeds Lord Mayor Sir Charles Lupton (1855–1935).

Birley was knighted in 1949.