Osborn's company served with the Army of the Potomac earning high marks and he was promoted to captain, major and colonel.

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.

After Osborn's military service ended, he was appointed assistant commissioner for the Bureau of Refugees, Freedmen and Abandoned Lands as part of Reconstruction in Florida in 1865 and 1866.

He is credited with being instrumental in passing legislation to complete construction of the Washington Monument[2] (which had been halted since before the Civil War).

He served as the U.S. commissioner at the Centennial Exposition in Philadelphia, Pennsylvania in 1876, the first official world's fair in the United States.