- oct
- oct(o)
- octad
- octadecanoic acid
- octagon
- octagonal
- octal
- octal notation
- octal number system
- octal numeration system
- octane
- octane number
- octane rating
- octanedioic acid
- oyster fungus
- oyster plant
- openside plane
- open_end wrench
- Osco_Umbrian
- omnipresent
- omnipresence
- old dominion
- overturn
- old dominion state
- oddball
- ortygan
- oodles
- omega
- ox
- order
- organic compound
- oculus
- out of sight
- out of mind
- orb
- optic
- ophthalmic
- on doorstep
- on coat tails
- oscitant
- open
- on the high side
- overdrive
- oxbridge
- oxide
- oxyacetylene
- oxyacetylene torch
- oxidise
- oxidation
- oxidisation