Obdurate :---> stubborn.
Obsequious :---> servile, slavishly attentive
Obsession :---> complex enthusiasm
Obstinate :---> refraining, to resist
Obviate :---> make unnecessary, get rid of.
Occlude :---> shut, close.
Officious :---> meddlesome
Onerous :---> burdensome.
Onus :---> impediment, liability, responsibility.
Oppression :---> tyranny, exploitation
Opprobrium :---> infamy, vilification
Opulence :---> wealth, plenty, abundance
Ordain :---> command.
Orthodox :---> conventional or traditional
Ostentatious :---> pretentious, showy.
Outrage :---> hurt, insult.
Outraged :---> injure, shocked, enraged.
Ovation :---> applause.
Ovulate :---> fertilize.