Gwasanaethodd Oscar Harmer o Gaerdydd gyda 'Bataliwn Dinas Caerdydd' (16eg Bataliwn, Welsh Regiment) ac yn ddiweddarach gyda'r King's Shropshire Light Infantry. Tra gyda Bataliwn Dinas Caerdydd, Oscar oedd negesydd personol prif swyddog y bataliwn, Lt-Col. Frank Gaskell, ac roedd ef wrth ochr Gaskell pan gafodd yntau ei ladd gan gêl-saethwr. Enillodd Oscar y DCM ('Distinguished Conduct Medal') tra'n gwasanaethu gyda'r KSLI ym 1918.: dywed y dystysgrif "For most conspicuous gallantry and devotion to duty. In the attack on Fresnoy-le-Petite, 19th September, 1918, when all the officers of his company had become casualties, he reorganised the company and took command. Later, with half a platoon and one Lewis gun, he captured an enemy post, from which he silenced a hostile machine-gun with Lewis gun fire, and succeeded in keeping down the fire of enemy snipers. The post was repeatedly attacked by the enemy with bombs and rifle grenades, but chiefly due to his fine example and energy all attacks were repulsed, though nearly all his men were wounded. Eventually he successfully withdrew his men under cover of darkness".