- obdurate: Stubbornly persistent or determined; resistant to persuasion
- obfuscate: To confuse; to make obscure
- obstinate: Unreasonably persistent or stubborn
- olfactory: Relating to the sense of smell
- oligarchy: A government in which a small group holds all of the control
- onus: A burden or obligation
- opine: To express an opinion or feeling
- opportunist: One who takes advantage of any chance to achieve an end, with little regard for morals or principles
- opprobrious: Disgraceful; shameful
- ornery: Having an irritable disposition or mood; cantankerous
- oscillate: To swing back and forth like a pendulum; to vary between opposing beliefs or feelings
- ossify: To change into bone; to become hardened or set
- ostracize: To exclude or reject from a group by common consent
- oust: To remove a title or position by force; eject