| Obeisance | Respect, Deference, Homage, Reverence, Honour |
| Obligatory | Compulsory, Required |
| Oblivion | Obscurity, Darkness, Blankness, Nothingness, Extinction |
| Obnoxious | Revolting, Repulsive, Repugnant, Disgusting, Offensive |
| Obsolete | Out-Date, Antiquated, Archaic, Ancient |
| Obstinate | Stubborn, Persistent, Recalcitrant, Tenacious, Headstrong |
| Obvious | Conspicuous, Definite |
| Officious | Dictatorial, Intrusive, Meddlesome, Interfering, Obtrusive |
| Omen | Premonition, Sign |
| Omit | Exclude, Remove |
| Opportune | Advantageous, Auspicious |
| Opulent | Rich, Wealthy, Affluent, Prosperous |
| Ostentatious | Showy, Flaunting, Boastful, Vain, Vainglorious, Flamboyant |
Need help? Leave a comment...