obese :: fat

oblique :: slanting; deviating from the perpendicular or from a straight line

obnoxious :: offensive

obsequious :: slavishly attentive; servile; sycophantic

obsolete :: outmoded

obtrusive :: pushing toward

obtuse :: blunt; stupid

occult :: mysterious; secret; supernatural

odious :: hateful

odoriferous :: giving off an order

odorous :: having an odor

officious :: meddlesome; excessively trying to please

olfactory :: concerning the sense of smell

ominous :: threatening

omnipotent :: all-powerful

omniscient :: all-knowing

omnivorous :: eating both plant and animal food; devouring everything

onerous :: burdensome

opalescent :: iridescent

opaque :: dark; not transparent

opportune :: timely; well chosen

opprobrious :: disgraceful

ornate :: excessively decorated; highly decorated

ostensible :: apparent; professed; pretended

ostentatious :: showy; pretentious

overt :: open to view