- OTR
- OTS
- OTSG
- Otsuka Museum of Art
- Ottaviano Petrucci
- Ottawa
- Ottawa Citizen
- Ottawa International Animation Festival
- 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