Why did William Sydney Porter choose to use the pen name O. Henry?

While in prison, Porter started to write short stories to earn money to support his daughter Margaret. His first work, 'Whistling 's Christmas Stocking' (1899), appeared in McClure's Magazine. The stories of adventure in the U.S. Southwest and in Central America gained an immediately success among readers. After doing three years of the five years sentence, Porter emerged from the prison in 1901 and changed his name to O. Henry. According to some sources, he acquired the pseudonym from a warder called Orrin Henry. It also could be an abbreviation of the name of a French pharmacist, Eteinne-Ossian Henry, found in the U.S. Dispensatory, a reference work Porter used when he was in the prison pharmacy.