| obliquus | oblique, obliquity |
| obliviscor | oblivion, oblivious |
| obscenus | obscene, obscenity |
| obscurus | obscure, obscurity |
| occupo | see capio |
| octo | octave, octavo |
| oculus | antler, binocular, eyelet, inoculate, inoculation, inveigle, monocle, ocular, oculist |
| odi | annoy, annoyance, annoying (adj.), ennui, noisome, odious |
| odor | odoriferous, odorous |
| offendo | inoffensive, offend, offender, offense, offensive, unoffending; see also defendo |
| oleo | olfactory, redolent |
| omen | abominable, abomination, ominous, ominously |
| omnis | omnipotence, omnipotent, omnipresent, omniscience, omniscient, omnivorous |
| onus | onerous |
| opacus | opaque |
| operio | cover, coverlet, covert, covertly, coverture, curfew, discover, discoverer, discovery, handkerchief, kerchief, rediscover, uncover, undiscover |
| opinor | opine, opinion |
| ops | copious, copiously, copy, copybook, copyist, copyright, cornucopia, office, officer, official, officially, officiate, officious, opulence, opulent, unofficial |
| optimus | optimism, optimist, optimistic |
| opto | adopt, adoption, option, optional |
| opus | cooperate, coordination, disorder, disorderly, extraordinarily, extraordinary, inordinate, inordinately, insubordinate, ordain, order, orderly, ordinance, ordinarily, ordinary, ordination, ordnance, preordain, subordinate, subordination, sell-ordered |
| orbis | exorbitant, orb,orbit |
| ordior | primordial |
| ordo | coordinate, coordination, disorder, disorderly, extraodinarily, extraordinary, inordinate, inordinately, insubordinate, ordain, order, orderly, ordinance, ordinarily, ordinary, ordination, ordnance, preorcdain, subordinate, subordination, well-ordered |
| orior | aboriginal, aborigines, abortion, abortive, orient, oriental, orientation, origin, origianl, originality, originally, originate, originator |
| orno | adorn, adornment, ornament, ornamental, ornamentation, ornate, suborn |
| oro | adorable, adoration, adore, inexorable, inexorably, oracle, oracular, oration, oratorial, oratorio, oratory, orison, peroration |
| os (oris) | oral, orifice, usher |
| os (ossis) | osprey, ossify |
| otium | negotiate, negotiation |
| ovo | ovation |
| ovum | oval, ovary, ovule |
