O'Donnell was commissioned as a lieutenant into the West Yorkshire Regiment on 10 May 1882.[1]
He was promoted to captain in April 1890[2] and in June 1891 was made an adjutant.[3]
As the Second Boer War was going on, he was promoted to major in April 1901[4] Although this was later antedated to June 1900.[5]
In February 1908 he succeeded Herbert Watts as commanding officer of a battalion of the West Yorks, for which he was raised to the rank of lieutenant colonel.[6]
After four years in command he was placed on half-pay in February 1912.[7] He was also promoted to colonel, backdated to 4 October 1911.[8] In August he reverted to normal pay when he took command of a brigade of the Territorial Force.[9] He was moved to command a district in May 1914.[10]
He saw action during the First World War, became an authority on military training and wrote significant papers entitled "Catechism on Field Training"[11] and "Lectures to Recruits: The training of the soldier, a lecture to recruits, and Intercommunication and passing orders and messages".[12] Promoted in June 1915 to temporary brigadier general,[13] he became commander of the 106th Infantry Brigade on 13 May 1916[15] and briefly commanded the 35th Division on an acting basis from 17 September 1916 to 23 September 1916. He was appointed a Companion of the Order of St Michael and St George in the 1919 Birthday Honours.[17]