Oscar F. Johnston was the son of Theodore P. Johnston, of Wytheville, Virginia, who moved to Tennessee by 1845 and to Walker County, Georgia, where by 1850 he worked as a tanner. Oscar was appointed to the United States Naval Academy in 1846. He served in the U.S. Navy until 1861, spending some of this time on the La Plata River Basin Expedition together with his friend, Perry M. DeLeon. He resigned his commission in 1861 and joined the Confederate Navy, serving until 1865. After the war, Johnston became a partner in the Johnston and DeLeon Cotton and Commission Merchants in Savannah, Georgia. He divided his time between Ringgold, near his father's home in north Georgia, and his Savannah business.

In 1862 Johnston married Mary A. Irvin Leet, daughter of Judith Scott Branch and the Reverend Arthur Irvin Leet, who had a tanyard downstream from their plantation house, Beaumont, in the same area of Walker County. Mary Leet Johnston died in 1865. Johnston remarried, to Belle Billups of Watkinsville, Oconee County, Georgia, in 1877. He continued to assist his father and unmarried and widowed sisters in Walker County while also maintaining a home in Watkinsville. He and Belle had at least three children, Edward P., Martha Johnston White, and Julia L. Johnston, before Oscar died in 1898. He is buried in Watkinsville Cemetery.

Personal, family, business, and naval correspondence. Major correspondents who issued naval orders include Confederate President Jefferson Davis; U.S. Secretary of the Navy Gideon Welles; and Confederate Secretary of the Navy Stephen Mallory. Business correspondence centers on Johnston's partnership after the Civil War in the Johnston and DeLeon Cotton and Commission Merchants in Savannah, Chatham County, Georgia. Also included are drawings of a ship's hold space (possibly the C.S.A. FIREFLY, which Johnston commanded).