After the start of the war, Osborn became the
lieutenant colonel of the
39th Illinois Infantry on October 11, 1861, and was promoted to
colonel the following year on January 1. He led the
regiment in several campaigns and battles in the
Eastern Theater. Osborn and his command saw action in the 1862
Valley Campaign against
Confederate forces under
Lt. Gen. Thomas Jonathan "Stonewall" Jackson, participating in the
Battle of Port Republic on June 9. From July until September 1863, Osborn took part in Union operations against
Charleston, South Carolina, including attacks on
Fort Wagner and
Fort Sumter. In 1864, Osborn commanded the 1st
Brigade, 1st
Division of the
XXIV Corps of the
Army of the James. Osborn was badly wounded at the
Battle of Drewry's Bluff on May 14, 1864, when a musket ball shattered his right elbow and lodged in his arm. He stayed in the hospital until September before being released for duty. In December, he had recovered enough to report for duty. However, he suffered from
ankylosis of the injured elbow for the rest of his life. During the
Siege of Petersburg in 1864 into 1865, Osborn led a brigade in the XXIV Corps. He was
brevetted to the rank of
brigadier general on March 10, 1865. On April 2, 1865, Osborn's command was instrumental in the capture of Fort Gregg during the Union breakthrough, and he brevetted to
major general to rank from that date. and led the 1st Division of the XXIV Corps from May 2 – July 8. Osborn resigned from the Army on September 28. ==Postbellum career==