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- Obdurate - (adj.) showing unfeeling resistance to tender feelings; stubbornly persistent in wrongdoing
- Oblivion - (noun) the state of being disregarded or forgotten; total forgetfulness
- Obscure - (verb) make difficult to perceive by sight
- Obscure - (adj.) not clearly understood or expressed
- Obsequious - (adj.) attentive in an ingratiating or servile manner; attempting to win favor from influential people by flattery
- Obtuse - (adj.) lacking sharpness or intellectual ability
- Odyssey - (noun) a long wandering and eventful journey; a Greek epic poem (attributed to Homer) describing the journey of Odysseus after the fall of Troy
- Onerous - (adj.) not easily borne; wearing
- Onus - (noun) an onerous or difficult concern
- Opacity - (noun) the degree to which something reduces the passage of light; more obscure
- Opaque - (adj.) not clear; not transmitting or reflecting light or radiant energy; not clearly understood or expressed
- Opulent - (adj.) rich and superior in quality
- Oscillate - (verb) move or swing from side to side regularly; be undecided about something; waver between conflicting positions or courses of action
- Ostentatious - (adj.) intended to attract notice and impress others; of a display that is tawdry or vulgar
- Overt - (adj.) open and observable; not secret or hidden