- Omaha
- Oman
- Omani
- Omani monetary unit
- Omani rial
- Omar Bradley
- Omar Khayyam
- Omar Nelson Bradley
- omasum
- Omayyad
- OMB
- ombu
- ombudsman
- Omdurman
- omega
- Omega Centauri
- omega-3
- omega-3 fatty acid
- omega-6
- omega-6 fatty acid
- omelet
- omelet pan
- omelette
- omelette pan
- omen
- omental bursa
- omentum
- omeprazole
- omerta
- omicron
- ominous
- ominously
- omissible
- omission
- omissive
- omit
- Omiya
- ommastrephes
- ommatidium
- Ommiad
- omnibus
- omnidirectional
- omnidirectional antenna
- omnidirectional radio range
- omnidirectional range
- omnifarious
- omnipotence
- omnipotent
- omnipresence
- omnipresent
- omnirange
- omniscience
- omniscient
- omnium-gatherum
- omnivore
- omnivorous
- Omomyid
- Omomyid group
- omophagia
- Omotic
- omphalocele
- omphalos
- omphaloskepsis
- Omphalotus
- Omphalotus illudens
- omphalus
- Omsk