- overcharge
- orgasmic
- ornateness
- optical fiber
- oakum
- oddment
- overcoating
- overcoat
- order gymnophiona
- Old World vulture
- output_to_input ratio
- odds
- order of magnitude
- organ
- ONI
- Office of Intelligence Support
- OIS
- OSHA
- office staff
- olympian
- Onagraceae
- Oxalidaceae
- Onopordum
- Ozothamnus
- Onopordon
- Olearia
- oppression
- overstatement
- ostentation
- ostentatiousness
- outfall
- opposable
- oral communication
- oireachtas
- ozothamnus secundiflorus
- opera hat
- officer
- overhasty
- otoplasty
- oxyacid
- oxalacetic acid
- oil of vitriol
- oxaloacetic acid
- oxalic acid
- orthoboric acid
- onager
- ottoman
- ordovician period
- ore
- overweary