Wounded at Shiloh.
At the beginning of the Civil War, he enlisted as a private in the 41st Ohio Volunteer Infantry and was quickly promoted to captain. At the Battle of Shiloh, Opdycke, despite being wounded, picked up the regiment's fallen flag and led a charge that halted the Confederate advance. Governor Tod ordered Opdycke home, promoted him to colonel and asked him to recruit and command the 125th Ohio Volunteer Infantry. Throughout the rest of the war, this unit was known for its courage and tenacity, earning the name "Opdycke's Tigers."