| oblivion | obscurity |
| oblivious | inattentive or unmindful |
| obscure | dark, vague, unclear |
| obsequious | very attentive, fawning |
| obsessive | related to thinking about something constantly; |
| obstinate | stubborn, hard to control |
| obtuse | blunt, stupid |
| officious | meddlesome |
| ominous | threatening |
| provocative | arrousing anger or interest |