- on
- on a higher floor
- on a lower floor
- on a regular basis
- on air
- on all fours
- on an individual basis
- on an irregular basis
- on and off
- on approval
- on average
- on base
- on board
- on camera
- on earth
- on faith
- on fire
- on guard
- on hand
- on it
- on occasion
- on one hand
- on ones guard
- on paper
- on purpose
- on request
- on tap
- on that
- on that point
- on the average
- on the button
- on the coattails
- on the contrary
- on the dot
- on the face of it
- on the fence
- on the fly
- on the go
- on the hook
- on the job
- on the loose
- on the nose
- on the offensive
- on the one hand
- on the other hand
- on the Q.T.
- on the QT
- on the qui vive
- on the road
- on the side
- on the sly
- on the spot
- on the spur of the moment
- on the table
- on the way
- on the whole
- on the wing
- on time
- on tour
- on trial
- on your guard
- on-going
- on-key
- on-license
- on-line
- on-line database
- on-off switch
- on-site
- on-street
- on-the-job
- on-the-scene
- on-the-spot
- on/off switch
- onager
- Onagraceae
- onanism
- onanist
- once
- once again
- once and for all
- once in a while
- once more
- once-over
- onchocerciasis
- oncidium
- Oncidium papilio
- Oncidium papilio kramerianum
- oncogene
- oncologic
- oncological
- oncologist
- oncology
- oncoming
- Oncorhynchus
- Oncorhynchus keta
- Oncorhynchus kisutch
- Oncorhynchus nerka
- Oncorhynchus tshawytscha
- Oncovin
- Ondaatje
- Ondatra
- Ondatra zibethica
- one
- one after another
- one and only
- one at a time
- one by one
- one C
- one dollar bill
- one hundred
- one hundred eighty
- one hundred fifteen
- one hundred fifty
- one hundred fifty-five
- one hundred five
- one hundred forty
- one hundred forty-five
- one hundred ninety
- one hundred one
- one hundred seventy
- one hundred seventy-five
- one hundred sixty
- one hundred sixty-five
- one hundred ten
- one hundred thirty
- one hundred thirty-five
- one hundred twenty
- one hundred twenty-five
- one iron
- one million million
- one million million million
- one of the boys
- one ones coattails
- one percent
- one shot
- one thousand
- one thousand million
- one thousand thousand
- one time
- one-and-one
- one-armed
- one-armed bandit
- one-billionth
- one-celled
- one-dimensional
- one-dimensional language
- one-dimensionality
- one-eared
- one-eighth
- one-eyed
- one-fifth
- one-flowered pyrola
- one-flowered wintergreen
- one-fourth
- one-half
- one-handed
- one-hitter
- one-horse
- one-humped
- one-hundred-millionth
- one-hundred-thousandth
- one-hundredth
- one-liner
- one-man
- one-man rule
- one-member
- one-millionth
- one-night stand
- one-ninth
- one-off
- one-on-one
- one-party
- one-person
- one-piece
- one-quadrillionth
- one-quarter
- one-quintillionth
- one-seed
- one-seeded
- one-seventh
- one-sided
- one-sidedly
- one-sixteenth
- one-sixth
- one-sixtieth
- one-sixty-fourth
- one-spot
- one-step
- one-ten-thousandth
- one-tenth
- one-third
- one-thirty-second
- one-thousandth
- one-time
- one-to-one
- one-trillionth
- one-twelfth
- one-upmanship
- one-way
- one-way light time
- one-way street
- one-winged
- one-woman
- one-year
- one-year-old
- Onega
- Oneida
- ONeill
- oneiric
- oneirism
- oneiromancer
- oneiromancy
- oneness
- onerous
- onerously
- onerousness
- onetime
- ongoing
- ONI
- onion
- onion bagel
- onion bread
- onion butter
- onion dome
- onion louse
- onion mildew
- onion plant
- onion roll
- onion salt
- onion smut
- onion stem
- onion thrips
- onion yellow dwarf
- onion yellow-dwarf virus
- onionskin
- Oniscidae
- Oniscus
- online
- onlooker
- only
- only if
- only too
- only when
- Ono
- Onobrychis
- Onobrychis viciaefolia
- Onobrychis viciifolia
- Onoclea
- Onoclea sensibilis
- Onoclea struthiopteris
- onomancer
- onomancy
- onomastic
- onomasticon
- onomastics
- onomatomania
- onomatopoeia
- onomatopoeic
- onomatopoeical
- onomatopoetic
- Onondaga
- Ononis
- Ononis repens
- Ononis spinosa
- Onopordon
- Onopordon acanthium
- Onopordum
- Onopordum acanthium
- Onosmodium
- onrush
- Onsager
- onset
- onshore
- onside
- onslaught
- onstage
- Ontario
- ontogenesis
- ontogenetic
- ontogeny
- ontological
- ontology
- onus
- onward
- onward motion
- onwards
- Onychium
- Onychogalea
- onycholysis
- Onychomys
- Onychophora
- onychophoran
- onychosis
- onymous
- onyx
- onyx marble
- onyxis