Please read the information about this list on User:SirJective/Parenthesis before working with it!
- O'Day (crater) - O'Day
- O My Father (hymn) - O My Father
- Oberkampf (Paris Metro) - Oberkampf
- Observation tower (ride) - Observation tower
- Obstruction (baseball) - Obstruction
- Occasional Conformity Act (1711) - Occasional Conformity Act
- Odéon (Paris Metro) - Odéon
- Offensive philosophy (American football) - Offensive philosophy
- Office of the Military Advisor to the Commonwealth Government (Philippines) (r) - Office of the Military Advisor to the Commonwealth Government
- Official Opposition Shadow Cabinet (Australia) - Official Opposition Shadow Cabinet
- Official Opposition Shadow Cabinet (Canada) - Official Opposition Shadow Cabinet
- Official Opposition Shadow Cabinet (UK) - Official Opposition Shadow Cabinet
- Offside law (football) - Offside law
- Ogawa, Ibararaki (Higashiibaraki) - Ogawa, Ibararaki
- Oh Brother, Where Art Thou? (soundtrack) (r) - Oh Brother, Where Art Thou?
- Olbers (crater) - Olbers
- Old Harrison County Courthouse (Texas) - Old Harrison County Courthouse
- Old Major (Animal Farm) - Old Major
- Old Post Office Building (Washington) - Old Post Office Building
- Old Trick New Dog (album) - Old Trick New Dog
- Old Woman River (Ontario) - Old Woman River
- Olde Cheshire Cheese (London pub) - Olde Cheshire Cheese
- Olivia (Buffy the Vampire Slayer) - Olivia
- Olpe (district) - Olpe
- Olympic athletics medalists (men) (r) - Olympic athletics medalists
- Olympic athletics medalists (women) (r) - Olympic athletics medalists
- Olympic Baseball Center (Athens) (r) - Olympic Baseball Center
- Olympic Baseball Centre (Athens) - Olympic Baseball Centre
- Olympic medalists in athletics (men) - Olympic medalists in athletics
- Olympic medalists in athletics (women) - Olympic medalists in athletics
- Olympic medalists in swimming (men) - Olympic medalists in swimming
- Olympic Softball Stadium (Athens) - Olympic Softball Stadium
- Olympus Spaceport (Orbiter sim) - Olympus Spaceport
- Omega, (Navigation System) (r) - Omega,
- On the Town with The League of Gentlemen (radio series) (r) - On the Town with The League of Gentlemen
- One From the Heart (album) - One From the Heart
- One pair (poker) - One pair
- One pair (Poker) (r) - One pair
- Onega (disambiguation) - Onega
- Opel RAK.1 (plane) - Opel RAK.1
- Open Range (movie) (r) - Open Range (r)
- Operation Gothic Serpent timeline (Somalia, 1993) (r) - Operation Gothic Serpent timeline
- Operation Tsunami (Canada) - Operation Tsunami
- Opiuchus (planet) (r) - Opiuchus
- Opposition House Leader (Canada) (r) - Opposition House Leader
- Optimize (magazine) - Optimize
- Opéra (Paris Metro) - Opéra
- Opus 64 No 1 (Chopin) (r) - Opus 64 No 1
- Opuwo (Constituency) - Opuwo
- Ordinary (heraldry) - Ordinary
- Oregon Ballot Measure 11 (1994) - Oregon Ballot Measure 11
- Oregon Ballot Measure 16 (1994) - Oregon Ballot Measure 16
- Oregon Ballot Measure 25 (2002) - Oregon Ballot Measure 25
- Oregon Ballot Measure 27 (2002) - Oregon Ballot Measure 27
- Oregon Ballot Measure 28 (2003) - Oregon Ballot Measure 28
- Oregon Ballot Measure 30 (2004) - Oregon Ballot Measure 30
- Oregon Ballot Measure 36 (1996) - Oregon Ballot Measure 36
- Oregon Ballot Measure 47 (1996) - Oregon Ballot Measure 47
- Oregon Ballot Measure 5 (1990) - Oregon Ballot Measure 5
- Oregon Ballot Measure 50 (1997) - Oregon Ballot Measure 50
- Oregon Ballot Measure 51 (1997) - Oregon Ballot Measure 51
- Oregon Ballot Measure 60 (1998) - Oregon Ballot Measure 60
- Oregon Ballot Measure 9 (1992) - Oregon Ballot Measure 9
- Oregon Delegation in 107th &/or 108th Congress (2001-2005) - Oregon Delegation in 107th &/or 108th Congress
- Organizing Committee of Principist Trotskyism (Fourth International) - Organizing Committee of Principist Trotskyism
- Organizing Committee, Communist Party of India (Marxist-Leninist) - Organizing Committee, Communist Party of India
- Orishas (band) - Orishas
- Orontius (crater) - Orontius
- Orsay Ville (Paris Metro) - Orsay Ville
- Orthosie (moon) - Orthosie
- Osteopetrosis, (generic term) - Osteopetrosis,
- Osterode (district) - Osterode
- Otter island (fictional) - Otter island
- Our Classbook (Twins album) (r) - Our Classbook
- Our Classbook (Twins) (r) - Our Classbook
- Ourcq (Paris Metro) - Ourcq
- Outpost (computer game) - Outpost
- Outsider (Known Space) - Outsider
- Oyggjaframi (m-l) - Oyggjaframi
- Oyggjaframji (m-l) (r) - Oyggjaframji