In 1864, Osborn commanded the 1st Brigade, 1st Division of the XXIV Corps of the Army of the James.[4] 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.[5]
Osborn (seated, left) and his staff during the American Civil War
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.[3] He was promoted to brigadier general in the United States Volunteers on May 1, 1865,[7] and led the 1st Division of the XXIV Corps from May 2 – July 8. Osborn resigned from the Army on September 28.[6]
Postbellum career
Osborn returned to his legal practice in Chicago, engaged in several business ventures, and he also held several federally appointed positions. He was elected as the treasurer of Cook County, Illinois, serving from 1867 until 1869.[3] He entered the State Department in 1874 after receiving a presidential appointment as the U.S. Minister to Argentina. He served in Buenos Aires until 1885.[8] He was significant involved in the negotiations to the Boundary treaty of 1881 between Chile and Argentina.[9]
In the spring of 1904, Osborn died in Washington, D.C., after suffering for ten hours from a massive cerebral hemorrhage that paralyzed him.
"Los respectivos signatarios deberíamos expresar, en este acto y en el nombre de sus respectivos gobiernos, el profundo aprecio hacia la benévola asistencia con la que sus Excelencias, los Ministros de Estados Unidos acreditados en la República Argentina y en la de Chile, el mayor general Thomas O. Osborn y Mr. Thomas A. Osborn, contribuyeron a las negociaciones que llevaron al acuerdo definitivo y que ha sido firmado gracias a ellos"'
References
Eicher, John H., and Eicher, David J., Civil War High Commands, Palo Alto, California: Stanford University Press, 2001, ISBN0-8047-3641-3.
Heidler, David S., and Heidler, Jeanne T., eds., Encyclopedia of the American Civil War: A Political, Social, and Military History, W. W. Norton & Company, 2000, ISBN0-393-04758-X.
U.S. War Department, The War of the Rebellion: a Compilation of the Official Records of the Union and Confederate Armies, U.S. Government Printing Office, 1880–1901.
Warner, Ezra J., Generals in Blue: The Lives of the Union Commanders, Louisiana State University Press, 1964, ISBN0-8071-0822-7.
Welsh, Jack D., Medical Histories of Union Generals, Kent State University Press, 2005, ISBN978-0-87338-649-4.
Brigadier general 
Brevet major general.png){kind=small}
