Oswald McLeod was the son of George Frederick McLeod and his wife, Elizabeth Derippe Williams.He was 18 years old when he enlisted and had to have the permission of his parents to sign up. At the time he enlisted, he was working on a farm at Elmore, Victoria, but had two years work with city retailers Foy and Gibson prior to this. He gave as his home address 'Airlie', 314 Albert St., East Melbourne, where his parents lived. It was also the office of the Interstate Commission, for whom his father worked.

Oswald McLeod had previous military experience, having had two years with the Senior Cadets at Northcote. He enlisted on 29 May, 1915, and after undergoing training was attached to the 8th Reinforcements, 4th Light Horse Regiment. The regiment had been formed as divisional cavalry, that is, not as a separate fighting unit like the Light Horse, but one in support of the troops in the trenches. Over 20% of them, like Oswald McLeod, were city dwellers, although he had experience working on a farm. They left for Egypt on board HMAT Kyarra A5 on 20 August, 1915, disembarking at Alexandria on 27 December that year. 

Once in Egypt, the 4th Divison recruits were placed in the 1st Anzac Mounted Regiment, their task being to defend the Nile Valley against pro-Turkish Arab forces. Then, in March. 1916, units B and D of the 1st ANZAC Mounted regiment were detached: Units A and C remained in Egypt, while units B and D left for the Western Front. Oswald McLeod was a member of B Unit. They sailed on HMS Castirian on 21 March, 1916, landing at Marseilles on  27 March. From here they left for the Western Front, joining with a Squadron from the Otago Mounted Rifles, a New Zealand unit, to form the 11 Australian Mounted Regiment. 

The nature of trench warfare limited the number of tasks mounted troops could fulfill. They were engaged in such activities as facilitating troop movement, traffic control, rear area security, escort of prisoners and reconnissance tasks. However, they became more immediately involved as the war entered its more mobile phase, which included the German retreat to the Hindenburg Line in early 1917 and the stemming of the German Spring Offensive of 1918 and the allied offensive of August-September, 1918.

Oswald McLeod took leave in France on 21 January, 1917, returning to the front on 13 February. He was detached from the 11 Anzacs to the NZ Divisional Corps in April, then rejoined his own unit on 5th May. From there, he was deployed in the Anti-Aircraft Section of the 11 Anzacs and seems to have moved in and out of his regiment and the Anti-Aircraft section four times between May and August. This would place him at he Battle of Messines in June, 1917, which moved the battle front from the Somme Valley in north-west France to the Ypres sector in Belgium. The 11 Anzac Mounted Regiment was heavily involved here, not as mounted troops, because the whole area was a quagmire, but serving in the trenches in a dismounted role, which would perhaps explain why Oswald Mc Leod was with the Anti Aircraft section.

Following Messines, Oswald McLeod was granted leave from 8 August, rejoining his unit on 1 September. In January, he was hospitalised by an unnamed illness, which took him out of the battlefront for three weeks. He must have been seen as still not fit for front-line duty because on 16 February, he was moved to the 22nd Salvage Corps, where he remained for another three weeks, before returning to the 11 Anzacs in Belgium, just in time to be in the middle of the German Spring Offensive.

He was killed in action 18 April 1918 at Mont Kemmel near Villers Brettoneux. A German counter-attack at Mt. Kemmel was part of the German Spring Offensive and the Australian troops resisting the attack suffered badly, with Oswald McLeod's 11 Anzac Mounted Regiment suffering more casualties in April 1918 than in the rest of the war combined. Oswald McLeod was just 21 when he died. He is buried at the Villers-Bretonneux Cemetery, Villers-Bretonneux, Picardie, France, and remembered in the Australian War Memorial Roll of Honour Cards 145 and on Panel 9 of the Commemorative Area.