Mystery Men & Women:

The O's

  Terry O'Brien: 1940, Fight Comics #1 (Fiction). Gang smasher policeman. In his first adventure he goes after an insurance ie protection racket. But, there's some other mystery at work in the story, as a group of six masked men seem to be a Secret 6 type of organization of concerned citizens and seem to be somehow responsible with putting O'Brien on the case.
  Dolly O'Dare: 1944, Scoop Comics #8 (Chesler). Dolly is a top sleuth for the police department. She has a notable criminal she strives against in Baron Blue.
  Pat O'Day: 1936, Detective Picture Stories #1 (Centaur). Pat is a city detective called to solve perplexing murders.
  Patty O'Day: 1939, Wonderworld #3 (Fox Features). Patty O'Day is one of those newsreel reporters who ends up making the news as much as reporting it. She and her assistant Ham are the "Comet" Newsreel team. She's romantically involved with a suave man of mystery called Lord Mike who turns out to be a secret agent. Ham is a good-natured two fisted fellow.
  Jerry O'Keefe: 1939, Keen Detective Funnies v2#11? (Centaur). State trooper Jerry O'Keefe runs afoul of a desperate gang of smugglers on a foggy night. This 2 issue adventure is apparently it.
  King O'Leary: 1944, Red Band Comics (Enwil/Rural); 1945, Triple Threat Comics (Gerona). Crack city editor and reporter and Kitty Allen is his top-notch photographer for the Daily Sentinel. Presumably, the King O'Leary reported to be in Triple Threat is the same character. Might even be the same story.
Spark O'Leary: 1939, Keen Detective Funnies v2#10? (Centaur). Spark is a radio newshawk and a halfway decent detective.
  Molly O'Moore: 1944, Terrific Comics #2 (Et-Es-Go/Holyoke). O'Moore teamed up with Scoop Scanlon to solve many crimes.
  Bobby O'Neill: 1939, Amazing Mystery Funnies (Centaur). Robert O'Neill is a patrolman on waterfront duty and his brother Micky is a newsreel cameraman. Rita May is a friend whose sister was ruined by dope peddlers and she killed them but is paroled in O'Neill's custody for turning state's evidence against the head smuggler Captain Dutch Silver. Done by Bill Everett.
  Ted O'Neil: 1940, Prize Comics #1 (Prize). O'Neil is an American fighter pilot and soldier of fortune who in the days before US involvement, joined the RAF to fight the Nazis
  Bonnie O'Toole: 1941, Fantastic Comics #22 (Fox). Bonnie is a debutante and when insurance scammers use her to fake an accident she does a bit of sleuthing on her own. She does so well exposing the racket, an insurance company hires her to be a detective for them.
  Ol' Skipper: 1946, Seven Seas Comics #1 (Leader Enterprises/Universal Phoenix Features). An old retired sea captain, he likes to spin his yarns for those that will listen. He lives in the beached ship “Pride O' Maine”. If his stories are to be believed, even as an old man he is vigorous and an amazingly strong swimmer.  In one tale, he reveals that his given name is David and he was married in his younger days. By tale's end, he takes in a white kitten named Furball for safekeeping as his audience in the person of a young girl had been told by her father to drown it.
  Ol' Skipper Webb: 1946, Seven Seas Comics #1 (Leader Enterprises/Universal Phoenix Features). A sea skipper that educates youngsters on useful things like tying knots. NOTE: Appears in informational strip called “Salty Stuff” and addressed as simply the Skipper or Ol' Skipper.  However, he's not the Ol' Skipper in the stories (that one has a long beard, this one is clean shaven). Assuming the character is meant to be the “author” of the strip which is labelled as “Ol' Skipper Webb”.
  Old Jacques: 1937, Detective Picture Stories #5 (Centaur). Old Jacques is a detective who is in pursuit of the master villain, the Wanderer for the past four years. The Wanderer roams the world in his crimes and is unpredictable leading Jacques to work with the French Surete, Scotland Yard and lastly the American police in his attempts to bring the crook to justice. The Wanderer has grown so confident, he's taken to goading Jacques by telling him what he plans to do next, daring him to stop him. Sadly, this is the only appearance of the villain and the elder sleuth. Old Jacques is an older man with a balding head, mustache and thorndyke beard but is still capable of quick thinking and quick action as in the one story he leaps the distance between private boxes at an opera house. What Jacques' actual official authority and commission in his pursuing the criminal is unrevealed though he talks with a French accent so he may be of the Surete.
Old Nick: ~1948, Pep Comics #66 (Schutters/MLJ). Old Nick is a dapper gentleman who is quick thinking and a decent detective. NOTE: While the publication information above is from a MLJ comic, Old Nick is an advertising character promoting the Old Nick candy bar by the same people that make Bit-O-Honey. I do not know how widespread the character was used. Other advertising not featuring the character for the candy bar in magazines are from the same time period.
Oracle
 Oracle: 1943, Startling #20 (Better). Private Detective Bob Paxton receives helpful visions whenever he sleeps or is knocked unconscious (which is disturbingly often the case). He's assisted by the pretty Lana Owens who addresses him by "Oracle" though he doesn't wear a mask or possess a secret identity.
  Oran of the Jungle: 1940, Fight Comics #1 (Fiction). Lost in the African jungles as a child, he grew up strong. So strong, that he attracted attention from slavers and unscrupulous boxing promotors. He travels to America to fight as a boxer and find his parents. Reunited with his father, he travels back to the jungles where he continues adventures as a boxing Tarzan. Eventually, the strip changed direction, the boxing angle was mostly dropped, Oran started looking a bit more like a youth, and his jungles were moved to another continent, Brazil was mentioned at one point.
  The Owl I: January 1940, Funny Pages #34 (Centaur). His day job was as a librarian with a passion for cleanliness named Jack and that's hinted at being an alias. His true passion is to help his dad get revenge for the loss of his legs due to some racketeers a decade earlier. To this end, his dad lives in seclusion in a shack in the swamps where he has inventeda winged flying suit and a portable radio that allows the two to communicate. Jack puts the suit to use to become the dreaded Owl.
the owlowl girl The Owl II: 1940, Crackajack Funnies #25 (Dell). Nick Terry is a special investigator for the police in Yorktown which is somewhere on the East Coast. He also fights crime as the hard hitting detective of the night, the Owl. Nick is apparently independently wealthy. He has an Asian servant named Soto who knows his secret. As the Owl, he has a themed Owlplane and speedboat, the Owlcraft. His cape hooks to his arms and allows him to glide. His girlfriend is Belle Wayne, gossip columnist/reporter. Shortly after he tells her his secret identity, she fashions a similar costume to fight crime alongside him (#32).