Big O
- Barack Obama
- Big "O"
- Big O (Ferris wheel)
- Big O Tires
- Big O notation
- Big Omega function (disambiguation)
- Big Orange Chorus
- Bigo (disambiguation)
- Glenn Ordway
- Oliver Miller
- Olympic Stadium (Montreal)
- Omaha–Council Bluffs metropolitan area
- Omar Gooding
- Omega
- Ontario Motor Speedway
- Oprah Winfrey
- Oscar McInerney
- Oscar Robertson
- Oscar Santana
- Otis Redding
- Roy Orbison
- Takashi Nagasaki
- The Big O
- The Big O (album)
- Time complexity