[O]

oaken adj. Made of or from oak.
oakum n. Hemp-fiber obtained by untwisting and picking out loosely the yarns of old hemp rope.
obdurate adj. Impassive to feelings of humanity or pity.
obelisk n. A square shaft with pyramidal top, usually monumental or commemorative.
obese adj. Exceedingly fat.
obesity n. Excessive fatness.
obituary adj. A published notice of a death.
objective adj. Grasping and representing facts as they are.
objector n. One who objects, as to a proposition, measure, or ruling.
obligate v. To hold to the fulfillment of duty.
obligatory adj. Binding in law or conscience.
oblique adj. Slanting; said of lines.
obliterate v. To cause to disappear.
oblivion n. The state of having passed out of the memory or of being utterly forgotten.
oblong adj. Longer than broad: applied most commonly to rectangular objects considerably elongated
obnoxious adj. Detestable.
obsequies n. Funeral rites.
obsequious adj. Showing a servile readiness to fall in with the wishes or will of another.
observance n. A traditional form or customary act.
observant adj. Quick to notice.
observatory n. A building designed for systematic astronomical observations.
obsolescence n. The condition or process of gradually falling into disuse.
obsolescent adj. Passing out of use, as a word.
obsolete adj. No longer practiced or accepted.
obstetrician n. A practitioner of midwifery.
obstetrics n. The branch of medical science concerned with the treatment and care of women during pregnancy.
obstinacy n. Stubborn adherence to opinion, arising from conceit or the desire to have one's own way.
obstreperous adj. Boisterous.
obstruct v. To fill with impediments so as to prevent passage, either wholly or in part.
obstruction n. Hindrance.
obtrude v. To be pushed or to push oneself into undue prominence.
obtrusive adj. Tending to be pushed or to push oneself into undue prominence.
obvert v. To turn the front or principal side of (a thing) toward any person or object.
obviate v. To clear away or provide for, as an objection or difficulty.
occasion n. An important event or celebration.
Occident n. The countries lying west of Asia and the Turkish dominions.
occlude v. To absorb, as a gas by a metal.
occult adj. Existing but not immediately perceptible.
occupant n. A tenant in possession of property, as distinguished from the actual owner.
occurrence n. A happening.
octagon n. A figure with eight sides and eight angles.
octave n. A note at this interval above or below any other, considered in relation to that other.
octavo n. A book, or collection of paper in which the sheets are so folded as to make eight leaves.
octogenarian adj. A person of between eighty and ninety years.
ocular adj. Of or pertaining to the eye.
oculist n. One versed or skilled in treating diseases of the eye.
oddity n. An eccentricity.
ode n. The form of lyric poetry anciently intended to be sung.
odious adj. Hateful.
odium n. A feeling of extreme repugnance, or of dislike and disgust.
odoriferous adj. Having or diffusing an odor or scent, especially an agreeable one.
odorous adj. Having an odor, especially a fragrant one.
off adj. Farther or more distant.
offhand adv. Without preparation.

officiate v. To act as an officer or leader.
officious adj. Intermeddling with what is not one's concern.
offshoot n. Something that branches off from the parent stock.
ogre n. A demon or monster that was supposed to devour human beings.
ointment n. A fatty preparation with a butter-like consistency in which a medicinal substance exists.
olfactory adj. of or pertaining to the sense of smell.
olive-branch n. A branch of the olive-tree, as an emblem of peace.
ominous adj. Portentous.
omission n. Exclusion.
omnipotence n. Unlimited and universal power.
Omnipotent adj. Possessed of unlimited and universal power.
omniscience n. Unlimited or infinite knowledge.
omniscient adj. Characterized by unlimited or infinite knowledge.
omnivorous adj. Eating or living upon food of all kinds indiscriminately.
onerous adj. Burdensome or oppressive.