Sitwell was born on 6 December 1892 at 3 Arlington Street, St James's, London.
His parents were Sir George Reresby Sitwell, fourth baronet, genealogist and antiquarian, and Lady Ida Emily Augusta (née Denison).
He grew up in the family seat at Renishaw Hall, Derbyshire, and at Scarborough, and went to Ludgrove School, then Eton College from 1906 to 1909.
For many years his entry in Who's Who contained the phrase "Educ[ated]: during the holidays from Eton."

In 1911 he joined the Sherwood Rangers Yeomanry but, not cut out to be a cavalry officer, transferred to the Grenadier Guards at the Tower of London from where, in his off-duty time, he could frequent theatres and art galleries.
Sitwell was born on 6 December 1892 at 3 Arlington Street, St James's, London. His parents were Sir George Reresby Sitwell, fourth baronet, genealogist and antiquarian, and Lady Ida Emily Augusta (née Denison). He grew up in the family seat at Renishaw Hall, Derbyshire, and at Scarborough, and went to Ludgrove School, then Eton College from 1906 to 1909. For many years his entry in Who's Who contained the phrase "Educ[ated]: during the holidays from Eton."
In 1911 he joined the Sherwood Rangers Yeomanry but, not cut out to be a cavalry officer, transferred to the Grenadier Guards at the Tower of London from where, in his off-duty time, he could frequent theatres and art galleries.
Osbert Sitwell