| A | B |
|---|
| mastodon | large, extinct, toothed animal |
| odontoid | toothlike |
| odontology | dentistry |
| orthodontics | the branch of dentistry for straightening teeth |
| octad | a series or group of eight |
| octagon | a plane figure with eight angles and eight sides |
| octal | of the number eight |
| octan | occurring every eighth day |
| octarchy | government by eight rulers |
| octet | any group of eight |
| octogenarian | a person between eighty and ninety years old |
| octopod | an animal with eight limbs |
| octuple | eightfold |
| omega | the last letter of the Greek alphabet |
| acronym | a word formed from the first letters of a word series |
| anonymous | with no name |
| antonym | a word with the opposite meaning of another word |
| homonym | a word with the same pronunciation but different spelling and meaning as another |
| pseudonym | a fictitious name |
| synonym | a word having the same or nearly the same meaning as another word |
| ophthalmia | inflammation of the eye |
| ophthalmology | the branch of medicine dealing with the eye |
| optic | the eye or the sense of sight |
| optician | one who prepares and dispenses eyeglasses |
| synopsis | a summary |
| ornithology | the branch of zoology dealing with birds |
| ornithosis | a virus disease transmitted by birds |
| orthodontics | the branch of dentistry for straightening teeth |
| orthodox | conforming to established doctrines |
| orthography | correct spelling |
| orthopedics | the branch of surgery dealing with bones, joints, and muscles |
