- opaque: dark; not transparent;
- opportunist: taking immediate advantage, often unethically, of any circumstance of possible benefit
- optimist: a person disposed to take a favorable view of things
- optional: left to one's choice; not required
- opulence: extreme wealth; luxuriousness; abundance
- ornate: elaborately decorated
- ostentatious: showy; pretentious; trying to attract attention
- pacifist: someone opposed to violence as a means of settling disputes
- painstaking: characterized by extreme care and great effort
- palliate: provide physical relief, as from pain
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