- original: Not copied nor produced by imitation.
- originate: To cause or constitute the beginning or first stage of the existence of.
- ornate: Ornamented to a marked degree.
- orthodox: Holding the commonly accepted faith.
- orthodoxy: Acceptance of the common faith.
- orthogonal: Having or determined by right angles.
- orthopedic: Relating to the correcting or preventing of deformity
- orthopedist: One who practices the correcting or preventing of deformity
- oscillate: To swing back and forth.
- osculate: To kiss.
- ossify: to convert into bone.
- ostentation: A display dictated by vanity and intended to invite applause or flattery.
- ostracism: Exclusion from intercourse or favor, as in society or politics.
- ostracize: To exclude from public or private favor.
- ought: To be under moral obligation to be or do.
- oust: To eject.
- out-and-out: Genuinely.
- out-of-the-way: Remotely situated.
- outbreak: A sudden and violent breaking forth, as of something that has been pent up or restrained.
- outburst: A violent issue, especially of passion in an individual.
- outcast: One rejected and despised, especially socially.
- outcry: A vehement or loud cry or clamor.
- outdo: To surpass.
- outlandish: Of barbarous, uncouth, and unfamiliar aspect or action.
- outlast: To last longer than.
- outlaw: A habitual lawbreaker.
- outlive: To continue to exist after.
- outpost: A detachment of troops stationed at a distance from the main body to guard against surprise.
- outrage: A gross infringement of morality or decency.
- outrageous: Shocking in conduct.
- outreach: To reach or go beyond.
- outride: To ride faster than.
- outrigger: A part built or arranged to project beyond a natural outline for support.
- outright: Entirely.
- outskirt: A border region.
- outstretch: To extend.
- outstrip: To go beyond.
- outweigh: To surpass in importance or excellence.
- overdo: To overtax the strength of.
- overdose: An excessive dose, usually so large a dose of a medicine that its effect is toxic.
- overeat: To eat to excess.
- overhang: A portion of a structure which projects or hangs over.
- overleap: To leap beyond.
- overlord: One who holds supremacy over another.
- overpass: To pass across or over, as a river.
- overpay: To pay or reward in excess.
- overpower: To gain supremacy or victory over by superior power.
- overproduction: Excessive production.
- overreach: To stretch out too far.
- overrun: To infest or ravage.