He was a member of Companies B and K of the 26th Regiment, Michigan Infantry during the Civil War, and died of illness 31 Dec 1864 in the Army Hospital at Alexandria, Virginia.
U.S. Civil War Soldiers, 1861-1865. Name: Oliver S. Barnum; Side: Union; Regiment State/Origin: Michigan; Regiment Name: 26 Michigan Infantry; Regiment Name Expanded: 26th Regiment, Michigan Infantry; Company: B, K; Rank In: Private; Rank In Expanded: Private; Rank Out: Private; Rank Out Expanded: Private; Alternate Name: Oliver/Barnum; Film Number: M545 roll 2.
In the 1860 US Census for Ashland Township, Newaygo, Michigan this family is listed with the Head of family shown as Alfred S. Barnum, as follows:
Household #264; Family #178
Barnum, Alfred S.; 26; M; Farmer; real property $200; personal property $40; b. New York
Barnum, Leana [sic]; 22; F; b. New York
Barnum, Wesley; 5; M; b. New York; attended school during previous year
Barnum, Amelia; 3; F; b. New York
Barnum, Alfred; 1; M; b. New York