Peter O'Brien (1842 – September 30, 1898) was an American soldier who fought in the American Civil War. O'Brien received his country's highest award for bravery during combat, the Medal of Honor. O'Brien's medal was won for capturing a Confederate flag, and an officer with his horse and equipment, at the Battle of Waynesboro, Virginia, on March 2, 1865. He was honored with the award on March 26, 1865.