Omega Phi Alpha
- A local men's fraternity founded in 1921 at University of the Pacific.
- http://www1.pacific.edu/student/opa/

Omicron Epsilon Pi Sorority, Incorporated
- A community-based, community service sorority for lesbian women founded February 2000 in Tallahassee, Florida
- http://www.oepi.com

Omega Pi Sorority
- Founded in 1985, and includes members from all universities in the Halifax, NS area.
- http://www.omegapisorority.com

Omicron Gamma Omega
- Local men's fraternity founded in 1966 at Millersville University.
- http://marauder.millersville.edu/~ogo/

Omicron Pi Omicron
- A local men's fraternity founded in 1903 at Clarkson University in Potsdam, New York.
- http://www.clarkson.edu/~opio/

Omega Sigma Tau
- Service and social fraternity established at UCLA in 1966.
- http://www.omegasigmatau.com/

Onyx
- African-American honor society founded in 1974 at the University of Pennsylvania. Overview, history, and member list.
- http://dolphin.upenn.edu/~onyxhs/