- Oakland Museum of California
- oakleaf goosefoot
- oak leaf poisoning
- Oak Ridge
- Oak Ridge National Laboratory
- O. A. Ladyzhenskaya
- O. A. Ladyzhenskaya (1922—2004)
- OANA
- OAO
- OAO-3
- oarfish
- oar from Hemudu
- Oars for the Boat of Art
- OAS
- oasis
- oasis in China
- oasis town
- oasis wet island effect
- O association
- OAT
- oat covered smut
- oat crown rust
- oat diseases
- oat flake
- Oath of Alliance