Oscar F. Washburn was born around 1841 in Canaan, New Hampshire, to Luther and Laura Washburn. His father was a farmer who owned $900 of real estate by 1850. His father died in the 1850s. Washburn grew up and attended school in Canaan, and by 1860, he was working as a farmhand. He married Harriet Fowler on January 5, 1861.
He enlisted in the Union army on March 12, 1862, and he mustered in as a private in Company D of the 4th New Hampshire Infantry six days later. The regiment took part in the Battle of Fort Wagner and the Bermuda Hundred campaign. He eventually earned a promotion to corporal. He reportedly “contracted diarrhaea at Bermuda Hundred” in July 1864 as a result of “inclement weather, exposure, night air +c.” He died of disease in Fortress Monroe, Virginia, on August 13, 1864.