| occluded | Stopped up; closed off; obstructed | |
| odious | extremely unpleasant; repulsive | |
| officious | Assertive of authority in an annoyingly domineering way, especially with regard to petty or trivial matters | |
| omission | The act of leaving out or neglecting to include something | |
| opaque | Not able to be seen through; not transparent | |
| opine | To express an opinion | |
| opportune | Well-chosen or particularly favorable or appropriate | |
| opulent | ostentatiously rich and luxurious or lavish | |
| orthodox | Adhering to the accepted or traditional and established faith, especially in religion | |
| ossified | Turned into bone; become bony | |