| Term | Definition |
| obstreperous | boisterous; unruly |
| obtrude | to push something toward or upon a person |
| obtuse | slow to comprehend |
| obviate | to prevent |
| occidental | western; opposite of oriental |
| occlude | to close; to shut; to block out |
| occult | hidden; secret; mysterious |
| ocular | pertaining to sight |
| odious | disgusting; hateful |
| odoriferous | giving off a displeasing or strong smell |
| odyssey | a long journey |
| offal | garbage; waste parts |
| officious | meddling; interfering |
| ogle | to look at with desire |
| ogre | monster; hideous being |
| olfactory | pertaining to smell |
| oligarchy | government in which power is in the hands of only a few individuals |
| Olympian | majestic |
| omen | an event which indicates the future |
| ominous | threatening; indicating evil or harm |