| 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. |