A rule established with any fairly common name is that there has to be someone by that name who was a member of a San Francisco rock band at some time. True to form, a Jack O'Brien shows up in the land of clanging cable cars circa the late '60s, with the pounding of a Hammer all his own. Hammer, not to be confused with nail-driving acts such as jazz fusion pianist Jan Hammer or rapper MC Hammer, was a somewhat heavy rock band from the Bay Area that received the production and promotional push of the Fillmore's Bill Graham through one self-titled album.
Listeners interested in the careers of noted session musicians may recall Hammer as the band that drummer John Guerin was in back in the days when he was still just a drummer playing in bands and not a veritable session icon. No similar fame followed the other Hammer members: O'Brien, singer John DeRoberts, bassist Richie McBride or keyboardist Norman Landsberg. During 1973, O'Brien was in a group known as Wormwood Scrubs with drummer Richie Fontana, later a sidekick of guitarist Billy Squier. The second Hammer album, taped but never issued, is considered one of the "great lost rock sessions," significantly by people who have never actually heard it. O'Brien shares songwriting credits for a pair of songs on the Hammer side that did make it out of the recording tool shop.