At the start of the Civil War, Oglesby was appointed colonel of the 8th Illinois Volunteer Infantry Regiment on April 25, 1861, and was soon given command of the 1st Brigade, 1st Division, District of Cairo, Department of the Missouri, serving under the command of Ulysses S. Grant. He was a well liked commander known to his troops as "Uncle Dick". He commanded his brigade at the battles of Fort Henry and Fort Donelson and soon after was promoted to brigadier general (March 21, 1862). He commanded the 2nd Brigade, 2nd Division, Army of the Tennessee, during the Siege of Corinth. He was severely wounded in his chest and back at the Battle of Corinth in October 1862.