Hart Watertown bm.  With the American Civil War looming, Osborn did not practice law for long. After the First Battle of Bull Run in 1861, he entered the Union Army as lieutenant. From his home in Jefferson County, New York, Osborn raised a company for light artillery service which became known as Company (or Battery) D, First Regiment, New York Light Artillery.Osborn's company served with the Army of the Potomac earning high marks and he was promoted to captain, major and colonel. As major, Osborn served under Major General Oliver O. Howard in the XI Corps leading in exemplary fashion (although the XI Corps was routed in both the Battle of Chancellorsville and Battle of Gettysburg). Osborn commanded the corps' artillery brigade at Gettysburg, and he was involved in the defense of Cemetery Hill on July 2, 1863, when the position was attacked by troops of Maj. Gen. Jubal Early.Osborn transferred to the Western theater with Howard. He served as inspector general when Howard became commander of the Army of the Tennessee. He left a detailed account of Maj. Gen. William T. Sherman's March to the Sea.