The Lexicon entries below explore significant Holocaust-related events, topics, places and personalities and are based on Yad Vashem's Encyclopedia of the Holocaust.
- Oberg, Carl Albrecht
- Ocheret, Mikhail Yosifovich
- Odessa
- Oeuvre de Secours aux Enfants - OSE
- Office for Jewish Affairs
- Office of Special Investigations
- Ohlendorf, Otto
- Olt, Maria
- Olympic Games of 1936
- Oneg Shabbat
- Oradour-sur-Glane
- Oranienburg
- Ordnungspolizei
- Organization of Ukrainian Nationalists
- Organization Schmelt
- Organization Todt
- Ostbataillone
- Ostindustrie GMBH
- Overduijn, Leendert