Merriam-Webster's Collegiate Dictionary:
omnibus
omnicompetence
omnicompetent
omnidirectional
omnidirectional range
omnifarious
omnificent
omnipotence
omnipotent
omnipotently
omnipresence
omnipresent
omnirange
omniscience
omniscient
omnisciently
omnium-gatherum
omnivore
omnivorous
omnivorously
omolon
omphalos
omphaloskepsis
omsk
on
on a dime
on a plate
on a platter
on a roll
on a sudden
on account
on account of
on all cylinders
on all hands
on an even keel
on and off
on approval
on average
on background
on balance
on behalf of
on board
on call
on camera
on commission
on consignment
on deadline
on deck
on demand
on draft
on duty
on earth
on edge
on end
on even keel
on every hand
on faith
on file
on fire
on foot
on fumes
on guard
on hand
on her beam-ends
on hire
on hold
on ice
on line
on no account
on occasion
on offer
on one's deathbed
on one's doorstep
on one's ear
on one's feet
on one's hands
on one's haunches
on one's last legs
on one's mettle
on one's own
on one's own account
on one's own hook
on one's own initiative
on one's own terms
on one's part
on one's sleeve
on one's toes
on one's uppers
on one's way
on order
on pain of
on paper
on pins and needles
on purpose
on record
on report
on sale
