| obdurate | hardened and unrepenting stubborn inflexible |
| obfuscate | to darken make obscure muddle |
| oblivious | unaware having no memory |
| obloquy | abusively detractive language sharp criticism vituperation |
| obsequious | too eager to obey or serve |
| obstreperous | noisy loud |
| obtain | to be established accepted or customary |
| obtrusive | projecting prominent undesirably noticeable |
| obtuse | blunt/stupid |
| obviate | to make unnecessary get rid of |
| occluded | blocked up |
| odious | repulsive hateful |
| odium | contempt dislike aversion |
| odor | smell favor reputation |
| officious | too eager or ready to help offer advice |
| ominous | threatening |
| onerous | needing effort burdensome |
| opaqueness | dullness/not allowing light to pass through |
| opprobrious | showing scorn or reproach |
| ossify | to turn to bone to settle rigidly into an idea |
| ostensible | seeming appearing as such professed |
| ostentation | display to obtain admiration or envy |
| ostracism | shut out from society refuse to meet talk |
| overhaul | examine thoroughly to learn about the condition |
| overweening | presumptuously arrogant overbearing immoderate being a jerk |
