[2] His father, General Tench Tilghman (a graduate of West Point military academy), and his mother was a daughter of John Leeds Kerr, United States Senator of Maryland from 1841 to 1843.

[4] When the American Civil War began in 1861, he volunteered as a Private in Company B, in Terry's Texas Rangers, of the Confederate States of America army.

[2] He became a Lieutenant and served as an aide on the staff of his cousin, General Lloyd Tilghman[5] (who was killed in front at the Battle of Champion Hill).

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.

[5] When the conflict between North and South had been brought to a close in 1865, Tilghman returned to native area of Talbot County, Maryland, and began his preparation for the legal profession.

Maryland Governor William Thomas Hamilton, appointed Oswald Tilghman, in 1881, as one of the two commissioners, with the rank of colonel, to represent the state at the Centennial Celebration of Cornwallis's surrender at Yorktown.