Oscar E. Pratt was born on December 17, 1838, in Ontario, New York, to Alva Pratt and Ann Peck. His father was a farmer who owned $4,000 of real estate and $2,300 of personal property by 1860. The family eventually moved to Webster, New York, and by 1860, Pratt was working as a farm laborer there. He enlisted in the Union army on August 13, 1862, and he mustered in as a private in Battery C of the 4th New York Heavy Artillery later that day. According to his service records, he was 5 feet, 8 inches tall, with light hair and blue eyes.
He mustered out in October 1863 and received a commission as a captain in Company F of the 7th USCT Infantry. The regiment took part in the siege of Petersburg and the Appomattox campaign. He was promoted to major on October 13, 1864, to lieutenant colonel on November 23, 1864, and to colonel on March 13, 1865. He mustered out on October 13, 1866.
Pratt returned to Webster after the war. He married Amelia Holley, and they had at least three children: William, born around 1865; Alice, born around 1869; and Howard, born around 1876. He worked as a physician, and by 1870, he owned $2,500 of real estate and $3,500 of personal property. By 1880, they were living in Oneonta, New York. He applied for a federal pension in December 1888 and eventually secured one. By 1910, they were living in Girard, Michigan, and Pratt was working as a druggist. He died of a cerebral hemorrhage in Branch, Michigan, on July 20, 1920.