- 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.