Slagle enlisted in the Army as a private on 4 August 1862 in Manlius and was mustered into Company D of the 104th Illinois Infantry on 27 August 1862. He was promoted to corporal at an unknown time. On 2 July 1863, at the Battle of Elk River, he was among a group of ten volunteers including leader George K. Marsh, Reuben Smalley, John Shapland, Charles Stacey, Richard J. Gage, and Samuel F. Holland, that successfully captured a Confederate defensive fortification while under heavy fire. All ten men would eventually win Medals of Honor.[2][4][6] Slagle received his medal on 30 October 1897.[2]
Slagle's Medal of Honor citation reads:[3]
The President of the United States of America, in the name of Congress, takes pleasure in presenting the Medal of Honor to Private Oscar Slagle, United States Army, for extraordinary heroism on 2 July 1863, while serving with Company D, 104th Illinois Infantry, in action at Elk River, Tennessee. Private Slagle voluntarily joined a small party that, under a heavy fire, captured a stockade and saved the bridge.
— Russel A. Alger, Secretary of War
Slagle was mustered out of the Army on 6 June 1865 at Washington D.C.[6]