What is the real name of O'Henry?

William Sydney Porter, American short story writer and made his pen name of O'Henry synonymous with surprise endings. In a little more than one decade, he published more than two hundred stories in magazines and books, some of which are still well known one century later. His work:
The Rolling Stone
The Gentle Grafter
Sixes and Sevens
Roads of Destiny
The Gift of the Magi
The Ransom of the Chief
Source : enotes.com