The initial idioms of the letter O

Toodle-oo	O tempora! O mores!
oak	oaks
oar	oasis
oat	oat opera
oater	oath
Obamacare	He that cannot obey cannot command
object to	object
objection	obligate to
oblige by	oblige to do
oblige with	oblige
obliterate	obliterate from
obliterated	sink into oblivion
obno	obnoc
obsessed with	stubborn as a mule
obtain for	obtanium