| opaque | Not able to be seen through; not transparent | |
| omission | The act of leaving out or neglecting to include something | |
| officious | Assertive of authority in an annoyingly domineering way, especially with regard to petty or trivial matters | |
| odious | extremely unpleasant; repulsive | |
| occluded | Stopped up; closed off; obstructed | |
| obtrusive | Noticeable or prominent in an unwelcome or intrusive way | |
| obstreperous | Noisy and difficult to control | |
| obsequious | Excessively obedient or attentive to an ingratiating or servile degree | |
| obloquy | Strong public criticism or verbal abuse | |
| objective | A goal or target that is aimed for | |