O

Oaf awkward person

Obdurate unyielding, hardhearted

Obeisance homage, deference

Obelisk tall column, monument

Obese fat

Obfuscate bewilder, muddle

Obituary eulogy

Objective (adj.) unbiased

Objective (noun) goal

Objectivity impartiality

Oblation offering, sacrifice

Obligatory required, compulsory

Oblige compel

Obliging accommodating, considerate

Oblique indirect

Obliquity perversity

Obliterate destroy

Oblong elliptical, oval

Obloquy slander

Obscure vague, unclear

Obsequious fawning, servile

Obsequy funeral ceremony

Observant watchful

Obsolete outdated

Obstinate stubborn

Obstreperous noisy, unruly

Obtain gain possession

Obtrusive forward, meddlesome

Obtuse stupid

Obviate make unnecessary

Occident the West

Occlude block

Occult mystical, secret, relating to the supernatural or witchcraft

Octogenarian person in her eighties

Ocular optic, visual

Ode poem

Odious despicable

Odoriferous pleasant odor

Odyssey journey

Offal inedible parts of a butchered animal

Offertory church collection

Officiate supervise

Officious forward, obtrusive

Offset counterbalance

Ogle flirt

Ogre monster, demon

Oleaginous oily

Oligarchy aristocracy

Olio medley

Ominous threatening

Omnibus collection, compilation

Omnipotent all-powerful

Omniscient all-knowing

Onerous burdensome

Onslaught powerful attack

Ontology the study of the nature of existence

Onus burden

Opaque nontransparent

Operative working

Operetta musical comedy

Opiate narcotic

Opine think, express an opinion

Opportune well-timed, appropriate

Oppress persecute

Oppressive burdensome

Opprobrious abusive, scornful

Opprobrium disgrace

Oppugn assail

Opt decide, choose

Optimum best condition

Optional elective

Opulence wealth

Opus literary work or musical composition

Oracle prophet

Oration speech

Orator speaker

Orb sphere

Orchestrate organize

Ordain appoint

Orderly neat, arranged

Ordinance law

Ordnance artillery

Orient align, familiarize

Orison prayer

Ornate lavishly decorated

Ornithology study of birds

Orthodox conventional

Oscillate waver, swing

Ossify harden

Ostensible apparent, seeming

Ostentatious pretentious

Ostracize banish, shun

Otherworldly spiritual

Otiose idle

Ouster ejection

Outmoded out-of-date

Outré eccentric

Outset beginning

Ovation applause

Overrule disallow

Overture advance, proposal

Overweening arrogant, forward

Overwhelm overpower

Overwrought overworked, highstrung

Ovum egg, cell