Brian O'Brien was a former District Attorney who became the Clock in order to take a more proactive role in crime-fighting.
The Clock was an accomplished hypnotist, ventriloquist, pilot, boxer, chemist, inventor, investigator, safecracker, vocal mimic, and master of disguise, and after working completely solo from 1939 until 1940, was aided in his endeavors by ex-boxer 'Pug' Brady (starting in 1940), and by an orphaned girl named Butch Buchanan (starting in 1942). His secret identity was known only to these two aides, and to his father.
In his day, the Clock took down such dangerous public enemies as "Velvet" Marcon, "Silk" Basso, "Killer" Casca, "Gong Gong" Klanes and "Runt" Malay, Moe Klone and "Scrag" Scadone, "Scar" Sizza, the Screw Gang, the Skull Gang, Baldy "the Owl" Getzmore, the "Jay Bird", "the Asp", "the Devil" and "the Crab."
The Clock undeniably had a bit of a dark side. He once shot down an airplane, killing the pilot, then followed and shot at the parachuting tailgunner from that plane, then landed and found the bullet-wounded gunner and beat some information out of him, then went on to beat a confession out of the gunner's also-bullet-wounded boss. He once got a loan shark to confess, by sticking his supposedly-artificial hand into a blazing fireplace, revealing the real hand concealed within, then beating a written confession out of him. The Clock had a torture chamber in his sub-basement base of operations, and on at least one occasion used an iron maiden to extract information from a reluctant squealer. This chamber also had a medieval rack and a bed of hot coals., and a specially-built rat cage, with which the Clock once threatened to make a rat eat through one captive's abdomen.
He was also somewhat reckless, both driving and flying. He once deliberately caused a fatal head-on car collision in which two fleeing suspects were killed, by leaping out of his own car at the last split-second. In an aerial pursuit, O'Brien once flew his open-cockpit biplane into position over the getaway plane, put on his mask, climbed out on the wing, and dove onto the enemy plane, leaving his own plane to crash. The next time he tried this stunt, he brought a back-up pilot, who (as the fight developed) also jumped out, again allowing the Clock's plane to crash.
The Clock's long-time habit of leaving calling cards and witty notes at crime scenes and on arrested suspects backfired on him at least four times, as various criminals simply had fake "Clock" calling cards printed up, and left them at assorted crime scenes.
Like many stylish gentlemen of his era, Brian O'Brien smoked cigarettes, and carried them in an elegant cigarette case. In his Clock persona, he routinely carried some blank bullets around with him, in case an opportunity to reload a bad guy's gun with them should arise.
- Aviation 
- Chemistry He once was dosed with sodium di-nitrophenol, but he just happened to know the antidote for sodium di-nitrophenol poisoning (it's egg whites and milk), and was able to give himself a counterdose.
- Disguise He had an information-gathering underworld alter-ego as "Snowy" Winters.
- Driving He pulled off an extremely difficult and dangerous driving stunt and escaped alive while inflicting two fatalities.
- Gadgetry He invented a paralysis gun.
- Hypnosis He once hypnotized a guilty felon into visiting the police and telling on himself.
- Interrogation He routinely beat information and confessions out of perps of all sizes and ages.
- Investigation He was a persistent investigator, did some remarkably accurate guesswork, and kept a very complete set of crime files.
- Law As a former District Attorney, O'Brien has to know that most of his methods are completely illegal.
- Mimicry He typically not only knew the bad guys' passwords, but could say them in the same voice as the bad guys' bosses.
- Roomful of medieval torture instruments.
- Biplanes. Some of the biplanes pack machine-guns.
- Rarely, but not never, carries a gun.
- Has packed everything from tear gas to a trick walking-stick.
- This version of Brian O'Brien, including all history and corresponding appearances, was erased from existence following the collapse of the original Multiverse in the 1985–86 Crisis on Infinite Earths limited series. Even though versions of the character may have since appeared, this information does not apply to those versions.
- The Clock's home base is identified as New York City in Crack Comics #8 Dec 1940. The "Park Avenue" part of the address comes from the splash panel of his story in Crack Comics #22 Mar 1942.
- The Clock maintained multiple hideouts, in and around New York City.
- The Clock's first appearance can be attributed to either Funny Pages #6 or Funny Picture Stories #1 (both from Centaur Comics/Comics Magazine), as he appears in both issues and they share the same cover date of November 1936. The Clock appears on the cover of Funny Picture Stories #1, but not on that of Funny Pages #6.
- Quality Comics apparently later purchased The Clock from Centaur Comics. The Clock's first Quality appearance was Feature Funnies #3, December 1937. The series ran in Feature Comics until Feature Comics #31, April 1940, then moved to Crack Comics, where it ran from Crack Comics #1, May 1940, until Crack Comics #35, Autumn 1944.
- It is also possible that the Clock was owned by creator George E. Brenner (not Centaur Comics) and therefore Quality did not have to buy the character from Centaur, they just had to hire Brenner. At this late date, the truth is unknown.
- In 1992, when Malibu Comics revised many Centaur Comics characters in Protectors, they included the Clock, and DC Comics did not object. This has led to the general belief that the Clock is now in the public domain.
- In 2013, the Clock appeared in Masks from Dynamite Entertainment.
- Appearances of Brian O'Brien (Quality Universe)
- Images featuring Brian O'Brien (Quality Universe)
- Quotations by or about Brian O'Brien (Quality Universe)
- Character Gallery: Brian O'Brien (Quality Universe)
- Feature Comics #26
- Feature Comics #23
- Feature Funnies #3
- Crack Comics #7
- Crack Comics #6
- Feature Comics #22
- Feature Funnies #7
- Feature Funnies #12
- Feature Funnies #5
- Crack Comics #1
- Crack Comics #21
- Feature Funnies #10
- Feature Funnies #11
- Crack Comics #23
- Crack Comics #14
- Crack Comics #3
- Crack Comics #9
- Crack Comics #5
- Crack Comics #4
- Crack Comics #13
- Feature Funnies #4
- Feature Funnies #6
- Feature Funnies #9
- Feature Comics #31
- Feature Funnies #15
- Crack Comics #10
- Feature Comics #30
- Crack Comics #12
- Crack Comics #24
- Crack Comics #25
- Feature Comics #25