O. Henry, pseudonym of William Sydney Porter, original name William Sidney Porter(born Sept. 11, 1862, Greensboro, N.C., U.S.—died June 5, 1910, New York, N.Y.) American short-story writer whose tales romanticized the commonplace—in particular the life of ordinary people in New York City. His stories expressed the effect of coincidence on character through humour, grim or ironic, and often had surprise endings, a device that became identified with his name and cost him critical favour when its vogue had passed.