O'Hagan was a vendor in a detective novel Data was reading in early 2364 to become more prepared for a Dixon Hill adventure. (TNG: "The Big Goodbye")
This character was only mentioned in writing.
The pages seen in the episode were actually from the detective novel, The Listening Man, by author John A. Moroso. This novel was published in 1924. Information on O'Hagan was from pages 196-197.