- O Brother, Where Art Thou?: Danny Ocean, a dubious therapist, and Barton Fink have misadventures while running from Mr. Kruger. They get caught up with Klan member Sully and Ocean tries to reunite with his wife Elastigirl.
- O Lucky Man!: Alex DeLarge is a coffee salesman who meets several people on his travels, many several times over, including Captain Mainwaring as his foreman, a mayor and an African official; Jane Tennison as a free-spirited rich girl and a secretary; Captain Phillip Blumbertt as an interrogater and a Salvation Army Major; Andy Dalziel as an MC, his chauffeur, and a male nurse; Compo as two different policemen; Terry Collier as an examination doctor; and Boba Fett as a car crash victim, an experimental patient and a man with a sign.
- Ocean's Eleven and its sequels: Dr. Ross, Tyler Durden, and Jason Bourne join up with Gigli's younger brother, the manager of Hotel Rwanda and others to pull heists with the (sometimes questionable) help of
Julia Roberts, Velma Kelly, an Anglican Inquisitor, and the dad from Big Fish. Notable foes include a Corleone and Kirill.
- Ocean's 8: Scarlet Overkill leads a band of thieves including (or against) Catwoman (the third one), Lady Tremaine, Disgust, Bellatrix Lestrange and Bubble.
- The Astronaut version: Matt Kowalski, Roy McBride, Mark Watney, Tom Cooper and a bunch of others gather to commit high profile heists. A pity it's not in space. And one of their foes is Chuck Baker (French dub). Ryan Stone also gathers her own team to such ends, and Kowalski happens to be her brother.
- October Sky: Donnie Darko teams up with Chuck Sherman, Bullethead, and Jesse to win a science fair. Dr. Ellie Satler is their teacher.
- The Odd Life of Timothy Green: Jenna Rink and Tom Buchanan get themselves a very strange kid.
- Of Mice and Men: Lt. Dan and Cyrus "The Virus" Grissom are 30s migrant workers, but Cyrus is, well, slower than most folks. They both get work at a farm, where they deal with Audrey Horne, the lovely wife of farmer's son 3D. Also stars Mr. Hand as a fellow farmhand.
- Oldboy (American version): George "Dubya" Bush is trapped in a room for years.
- Old Dogs: The Genie, a werewolf, and Bolt suddenly have to take care of a pair of young twins. Their mother, Jetstream, and the girl are Bolt's real-life wife and daughter, but Bolt isn't the one personally involved with them.
- Oleander: Nausicaa has to deal with her breaking Speeching Evil Matriarch Catwoman (the first one) while drifting through several different foster homes, including Bridget Jones'.
- The Omen (1976): Atticus Finch discovers (by among others, the Second Doctor) that his foster son is the Antichrist. In the third film, Atticus' son grows up to be Dr. Alan Grant.
- Olympus Has Fallen: When terrorists led by Takeshi and take president Two-Face hostage, it's up to Leonidas, seeking redemption for failing to save Two-Face's wife Libby Parsons, to save him.
- On The Town: Don Lockwood and the original Danny Ocean have twenty-four hours to see New York City.
- Once Upon a Time in Hollywood: Calvin Candie and Aldo Raine are two guys out to make a name for themselves in Hollywood. As fate has it, Harley Quinn lives next door, and all are about to cross paths with Charles Manson. Ryu acts in kung fu movies.
- One Hour Photo: Popeye is a (nonromantic) Stalker with a Crush fixated on Lucilla's beautiful family (Non) Hilarity Ensues when he finds out they're not so perfect. With a special guest appearance by a mass-production EVA unit.
- One Night Stand (1997): Nino Brown cheats on his wife Chun-Li with Irena Gallier while visiting his friend Wayne Gale. Gale is dying of AIDS and Nino meets his brother Dale Cooper, who introduces him to his wife, who is none other than Irena!
- Only Lovers Left Alive: Loki loves Jadis 4ever (literally) and then he meets her sister Alice. With Prof. Broom, Mr. Chekov, and Agent Leiter.
- Oranges And Sunshine: Reba the blind photography chemist is a (questionable) social worker who meets Elrond and Faramir after she uprooted them from England and shipped them to Australia to endure orphanages of terror.
- Ordinary People: Alex Reiger is the psychiatrist for Nathan Ford, who fell into a depression after the death of his big brother. His parents are President Snow and Mary Richards.
- Oscar: Rocky makes a promise to his dying father Spartacus that he'll give up his life of crime, only to discover that the promise isn't so easy to keep after Dean Porter comes asking for his "daughter's" hand in marriage and his real daughter Mona Lisa Vito claims to be pregnant so she can marry her wayward boyfriend; it only gets more complicated from there. Features cameos by Mortimer Duke, Boon, Dave Kujan, Wadsworth, Walter Peck, Derek Smalls, the Master, Princess Aura, and Lily Munster. (It's just that kind of a movie.)
- The Other Boleyn Girl: The Girl with a Pearl Earring and her sister Queen Amidala (or Natasha Romanoff and Jane Foster, if you prefer) have issues with their king, the Incredible Hulk. Natasha's first husband is Sherlock Holmes, and her second is Marius. (And if you like bending history, their father is Thomas Cromwell.)
- The Other Guys: When Cowboy Cops Mace Windu and The Scorpion King are sidelined, lower-tiered cops Ron Burgundy and Leo Davidson step up to fight evil billionaire Damian Cockburn. With Agent Ajedrez as Ron's wife.
- The Other Woman (2014): Lawyer Fiona is having an affair with Jaime Lannister until she finds out he's married to Debbie. Then they find out he's cheating on both of them with Bernice... Steffie is Fiona's secretary, Kelly Severide is Debbie's brother, and Nash Bridges is Fiona's father.
- Our Family Wedding: Betty Suarez is getting married to Calvin Payne, much to her science teacher-dad's chagrin. Her future father-in-law is Idi Amin. Oh yeah, and Mike Chang is there too.
- Our Idiot Brother: Brian Fantana is the chilled-out hippie daydream dude to his type-A sisters Carley Bobby, Holly Shiftwell, and Trillian.
- The Outsiders: Preston Webb is a young outcast who lives with his brothers Johnny Castle and Benjamin Kane, but has to run away when his best friend Daniel LaRusso kills Leonard Unger. Preston's other friends are Trip Murphy, Colonel Claus von Stauffenberg, and Otto Maddox.
- Oz: The Great and Powerful: Harry Osborn is a magician who gets lost in a strange land and meets three witches: Lily of the West, Evie of the East, and Alma of the South. Also starring JD's voice and Joxer.
Role Association / Film O