- Ottawa River
- Ottawa River Solar Observatory
- otter
- otter board/trawl door
- Ottmar Mergenthaler
- Otto
- Otto Abraham
- Otto Brahm
- Otto Braun
- Otto cycle
- Otto Dix
- Otto Erich Deutsch
- Otto Friedrich von Gierke
- Otto Jespersen
- Otto Koloman Wagner
- Otto Lilienthal
- Otto Loewi
- Ottoman Empire
- Ottoman-Persian Wars
- Ottoman Public Debt Administration
- Otto Meyerhof
- Otto Neugebauer
- Otto Neurath
- Ottonian art
- Otto of Freising