In December 1862, Osborn was commissioned as a colonel in the 24th Massachusetts Infantry. He distinguished himself at the Battle of Tranter's Creek, North Carolina, in June 1862 by taking command of the Union forces. Osborn was in command of forces in the actions against Fort Wagner, and Fort Sumter, South Carolina from April 1862 until November 1864. In June 1864, his unit was called to Virginia during Richmond-Petersburg Campaign. Osborn mustered out in November 1864. He was brevetted to brigadier general after the end of the war for gallant service.
