O
- Daniel O'Connell Irish political figure,Lodge No. 189, Dublin, in 1797
- Dositej Obradović (1742–1811), Serbian author, philosopher, linguist, polyglot and the first minister of education of Serbia.
- Hans Christian Oersted (1777–1851), Danish physicist and chemist who discovered that electric currents create magnetic fields.
- Bernardo O'Higgins South American revolutionary leader and first Chilean head of state as Captain General
- Ransom E. Olds, automotive pioneer and founder of Oldsmobile. Capitol Lodge No. 66, Lansing, MI
- Shaquille O'Neal American professional basketball player, and entertainer. Made a Mason at Sight in Widow's Son Lodge No. 28 PHA, Boston, Massachusetts.
- William Onslow, 4th Earl of Onslow, British politician
- Camilo Osías, President of the Senate of the Philippines.
- William Dillon Otter, Canadian General. Initiated in Ionic Lodge, No. 25, Toronto in February 1869
Read more about this topic: List Of Freemasons