- 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.