oaken – Made of or from oak.
oakum – Hemp-fiber obtained by untwisting and picking out loosely the yarns of old hemp rope.
obdurate – Impassive to feelings of humanity or pity.
obelisk – A square shaft with pyramidal top, usually monumental or commemorative.
obese – Exceedingly fat.
obesity – Excessive fatness.
obituary – A published notice of a death.
objective – Grasping and representing facts as they are.
objector – One who objects, as to a proposition, measure, or ruling.
obligate – To hold to the fulfillment of duty.
obligatory – Binding in law or conscience.
oblique – Slanting; said of lines.
obliterate – To cause to disappear.
oblivion – The state of having passed out of the memory or of being utterly forgotten.
oblong – Longer than broad: applied most commonly to rectangular objects considerably elongated
obnoxious – Detestable.
obsequies – Funeral rites.
obsequious – Showing a servile readiness to fall in with the wishes or will of another.
observance – A traditional form or customary act.
observant – Quick to notice.
observatory – A building designed for systematic astronomical observations.
obsolescence – The condition or process of gradually falling into disuse.
obsolescent – Passing out of use, as a word.
obsolete – No longer practiced or accepted.
obstetrician – A practitioner of midwifery.
obstetrics – The branch of medical science concerned with the treatment and care of women during pregnancy.
obstinacy – Stubborn adherence to opinion, arising from conceit or the desire to have one’s own way.
obstreperous – Boisterous.
obstruct – To fill with impediments so as to prevent passage, either wholly or in part.
obstruction – Hindrance.
obtrude – To be pushed or to push oneself into undue prominence.
obtrusive – Tending to be pushed or to push oneself into undue prominence.
obvert – To turn the front or principal side of (a thing) toward any person or object.
obviate – To clear away or provide for, as an objection or difficulty.
occasion – An important event or celebration.
Occident – The countries lying west of Asia and the Turkish dominions.
occlude – To absorb, as a gas by a metal.
occult – Existing but not immediately perceptible.
occupant – A tenant in possession of property, as distinguished from the actual owner.
occurrence – A happening.
octagon – A figure with eight sides and eight angles.
octave – A note at this interval above or below any other, considered in relation to that other.
octavo – A book, or collection of paper in which the sheets are so folded as to make eight leaves.
octogenarian – A person of between eighty and ninety years.
ocular – Of or pertaining to the eye.
oculist – One versed or skilled in treating diseases of the eye.
oddity – An eccentricity.
ode – The form of lyric poetry anciently intended to be sung.
odious – Hateful.
odium – A feeling of extreme repugnance, or of dislike and disgust.
odoriferous – Having or diffusing an odor or scent, especially an agreeable one.
odorous – Having an odor, especially a fragrant one.
off – Farther or more distant.
offhand – Without preparation.
officiate – To act as an officer or leader.
officious – Intermeddling with what is not one’s concern.
offshoot – Something that branches off from the parent stock.
ogre – A demon or monster that was supposed to devour human beings.
ointment – A fatty preparation with a butter-like consistency in which a medicinal substance exists.
olfactory – of or pertaining to the sense of smell.
olive-branch – A branch of the olive-tree, as an emblem of peace.
ominous – Portentous.
omission – Exclusion.
omnipotence – Unlimited and universal power.
Omnipotent – Possessed of unlimited and universal power.
omniscience – Unlimited or infinite knowledge.
omniscient – Characterized by unlimited or infinite knowledge.
omnivorous – Eating or living upon food of all kinds indiscriminately.
onerous – Burdensome or oppressive.
onrush – Onset.
onset – An assault, especially of troops, upon an enemy or fortification.
onslaught – A violent onset.
onus – A burden or responsibility.
opalescence – The property of combined refraction and reflection of light, resulting in smoky tints.
opaque – Impervious to light.
operate – To put in action and supervise the working of.
operative – Active.
operator – One who works with or controls some machine or scientific apparatus.
operetta – A humorous play in dialogue and music, of more than one act.
opinion – A conclusion or judgment held with confidence, but falling short of positive knowledge.
opponent – One who supports the opposite side in a debate, discussion, struggle, or sport.
opportune – Especially fit as occurring, said, or done at the right moment.
opportunist – One who takes advantage of circumstances to gain his ends.
opportunity – Favorable or advantageous chance or opening.
opposite – Radically different or contrary in action or movement.
opprobrium – The state of being scornfully reproached or accused of evil.
optic – Pertaining to the eye or vision.
optician – One who makes or deals in optical instruments or eye-glasses.
optics – The science that treats of light and vision, and all that is connected with sight.
optimism – The view that everything in nature and the history of mankind is ordered for the best.
option – The right, power, or liberty of choosing.
optometry – Measurement of the powers of vision.
opulence – Affluence.
opulent – Wealthy.
oral – Uttered through the mouth.
orate – To deliver an elaborate or formal public speech.
oration – An elaborate or formal public speech.
orator – One who delivers an elaborate or formal speech.
oratorio – A composition for solo voices, chorus, and orchestra, generally taken from the scriptures.
oratory – The art of public speaking.
ordeal – Anything that severely tests courage, strength, patience, conscience, etc.
ordinal – That form of the numeral that shows the order of anything in a series, as first, second, third.
ordination – A consecration to the ministry.
ordnance – A general name for all kinds of weapons and their appliances used in war.
orgies – Wild or wanton revelry.
origin – The beginning of that which becomes or is made to be.
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.
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.
out-of-the-way – Remotely situated.
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.
oversee – To superintend.
overseer – A supervisor.
overshadow – To cast into the shade or render insignificant by comparison.
overstride – To step beyond.
overthrow – To vanquish an established ruler or government.
overtone – A harmonic.
overture – An instrumental prelude to an opera, oratorio, or ballet.
overweight – Preponderance.