1861, he volunteered as a Private in Company B, in Terry's Texas Rangers, of the Confederate States of America army. On April 6–7, 1862, he participated in the Battle of Shiloh in Tennessee and in the campaigns about Richmond, Virginia. He became a Lieutenant and served as an aide on the staff of his cousin, General Lloyd Tilghman (who was killed in front at the Battle of Champion Hill). On March 14, 1863, Oswald took active part, with his battery, in that led to the destruction of the United States steam frigate USS Mississippi (1841), of which Admiral George Dewey was then executive officer, when Admiral Farragut's fleet attempted to pass the Confederate batteries. For his bravery on this occasion, he was commended by Lieutenant Colonel de Gournay, who commanded the left wing of the Confederate batteries. During the Siege of Port Hudson in Louisiana, Lieutenant Oswald Tilghman commanded the Rock City artillery of Nashville, Tennessee, a heavy battery on the banks of the Mississippi River and was the only one of the four officers of that battery who survived the siege. Lieut. Oswald Tilghman was taken captive during May 22 – July 9, 1863 at the Siege of Port Hudson, then was held prisoner at Johnson's Island in Sandusky, Ohio for 23 months until the conclusion of the war.