| Term | Definition |
| obstreperous (adj) | boisterous; noisy |
| obtrusive (adj) | pushing forward |
| obtuse (adj) | blunt; stupid |
| obviate (v) | make unnecessary; get rid of |
| occult (adj) | mysterious; secret; supernatural |
| odious (adj) | hateful |
| officious (adj) | meddlesome; excessively trying to please |
| ogle (v) | glance coquettishly at; make eyes at |
| ominous (adj) | threatening |
| omnipotent (adj) | all-powerful |
| omniscient (adj) | all-knowing; wise |
| omnivorous (adj) | eating both plant and animal food; devouring everything |
| onerous (adj) | burdensome, troubling |
| opaque (adj) | not transparent |
| opiate (n) | medicine to induce sleep or deaden pain |