Met lifelong friend Spencer Tracy in high school in Milwaukee
Joined the US Navy during World War I; saw no combat
Returned to Milwaukee, graduated high school and attended law school
While visiting relatives in NYC, cast as a dancer in the Broadway production "Adrienne"
Returned to Milwaukee to look after his ill father; resumed law studies; when father recovered, returned to NYC
Rented an apartment in Manhattan with Tracy; attended acting classes paid for with his veteran's stipend
Cast alongside Tracy in the science fiction play "R.U.R."
Joined a traveling stock theater company
Returned to Milwaukee at a girlfriend's urging; worked as insurance salesman; was soon back in NYC, acting on stage
Had an uncredited turn as a police detective in "Compliments of the Season"
Breakthrough film role, Hildy Johnson in "The Front Page"
Played a studio head in Victor Fleming's Hollywood satire "Bombshell", starring Jean Harlow
First film with James Cagney, Lloyd Bacon's "Here Comes the Navy"
Reteamed with Cagney in Howard Hawks' "Ceiling Zero"
Featured in Bacon's "Devil Dogs of the Air" and "The Irish In Us" with James Cagney
Played the priest opposite Cagney's gangster in "Angels With Dirty Faces", Michael Curtiz's iconic portrait of childhood friends who take different paths
Reteamed with Bacon and Cagney, playing a wisecracking screenwriter in the fast-moving Hollywood spoof "Boy Meets Girl"
Starred as the titular Notre Dame coach in "Knute Rockne, All American"; producers initially wanted Cagney for the role but Rockne's widow insisted upon O'Brien
Played Father Francis Duffy, the beloved chaplain of WWI's famed New York Irish regiment in "The Fighting 69th"
Starred with George Raft in the second film adaptation of the Prohibition-set "Broadway"
Played Major Chick Davis, a commander in charge of turning cadets into fighter pilots in the quasi-documentary "Bombardier"
Was star of the jungle-set WWII adventure "Marine Raiders"
Starred as a lawyer trying to keep his clients out of trouble on their honeymoon in the zany but predictable caper "Having Wonderful Crime"
Performed on several of early television's drama showcases including "Kraft Television Theater" and "Ford Television Theater" (both NBC then ABC)
Starred with Anne Jeffreys in the adventure "Riffraff", playing private eye Dan Hammer
Played Gramp, a lovable ex-vaudevillean who serves as guardian to "The Boy with Green Hair", an orphaned and tormented youngster played by Dean Stockwell
Starred in "Fighting Father Dunne" as a parish priest in 1900s St Louis trying to help the street urchin newsboys
As Father O'Hara in "The Fireball", runs the orphanage that Mickey Rooney's rollerskating champ flees
Featured in "The People Against O'Hara"; paired with Spencer Tracy, who had never starred in a film with O'Brien despite their longtime friendship
Played the Commander of a Navy warship leading an attack on Japan in "Okinawa", a WWII action drama
Co-starred in "Inside Detroit", a dramatized expose of corruption in the US auto industry
Starred with Tracy in John Ford's political drama "The Last Hurrah"
Had a featured role in the Billy Wilder classic "Some Like it Hot", playing the cop trailing the gangsters and the bumbling heros in drag
Starred as a maverick veteran lawyer partnered with his newly practicing by-the-book son in the ABC sitcom "Harrigan and Son"
Continued to actively perform on stage, working in regional touring theater productions with his wife Eloise Taylor and starring in a popular nightclub comedy act
Lone film credit of the decade, co-starring role in the Western "Town Tamer"
Starred as a retired Texas Ranger who reenters law enforcement to clean up a besieged small town in the ABC TV-movie "The Over-the-Hill Gang", co-starring such fellow veterans as Walter Brennan and Andy Devine
Starred as a doctor in "The Other Woman", an installment of of the daytime drama special series "ABC's Matinee Today"
Portrayed the US Vice President in the sequel/remake "Billy Jack Goes to Washington"
With Myrna Loy, co-starred as Burt Reynolds parents in the black comedy "The End"
Played Howard's uncle Joe on "Happy Days" (ABC)
Final TV-movie role, co-starring in the Gary Coleman vehicle "Scout's Honor"
Final film role, playing Delmas in Milos Forman's "Ragtime", also starring Cagney in his final role