Gouverneur K. Warren
In January 1862, Oliver joined the 12th New York Volunteer Infantry as a second lieutenant. During the war, he served as an aide to no fewer than four generals, including Daniel Butterfield, George Meade, Joseph Hooker and Gouverneur K. Warren. While he accepted a promotion to captain in April 1864, he declined further promotions. On March 8, 1865, President Abraham Lincoln nominated Oliver for appointment as a brevet brigadier general to rank from March 8, 1865 and the U.S. Senate confirmed the appointment on March 10, 1865. Oliver resigned his commission on May 6, 1865.