on |ɑːn| — in operation or operational

on-board |ˈɑːnˌbɔːrd| — (idiomatic) On or in a means of transportation.

on-license  — a license to sell liquor for consumption on the premises

on-line |ˈɒnˌlaɪn| — being in progress now

on-screen |ˈɔːnˈskrɪn| — As seen on a screen (as of television, film, or computer) rather than in real life.

on-site |ˈɑːnˈsaɪt| — taking place or located at the site

on-the-job |ˌɒn ðə ˈdʒɒb| — relating to or associated with a job or employment

on-the-spot  — at the scene of action

onager |ˈɑːnədʒər| — an engine that provided medieval artillery used during sieges; a heavy war engine for hurling large stones and other missiles

onagri |ˈɒnəɡraɪ| — plural form of onager

onanism |ˈəʊnənɪzəm| — manual stimulation of the genital organs (of yourself or another) for sexual pleasure

onboard |ˈɑːnˌbɔːrd| — Carried or used on or in a vehicle or vessel

once |wʌns| — on one occasion

once-over |ˈwʌnsˌəʊvə| — a swift cursory examination or inspection

oncer |ˈwʌnsər| — (informal, historical) A one-pound note.

oncological |ˌɒnkəˈlɒdʒɪkəl| — of or relating to or practicing oncology

oncologist |ɑːnˈkɑːlədʒɪst| — a specialist in oncology

oncology |ɑnˈkɑːlədʒɪ| — the branch of medicine concerned with the study and treatment of tumors

oncoming |ˈɑːnkʌmɪŋ| — the beginning or early stages

ondatra |ɒnˈdætrə| — muskrats

one |wʌn| — the smallest whole number or a numeral representing this number

one-armed |ˌwʌn ˈɑːrmd| — having one arm

one-dimensional |ˌwʌndaɪˈmenʃənəl| — relating to a single dimension or aspect; having no depth or scope

one-eyed |ˌwʌn ˈaɪd| — having or showing only one eye

one-fifth  — one part in five equal parts

one-fourth  — one of four equal parts

one-half  — one of two equal parts of a divisible whole

one-handed |ˈwʌnˌhændɪd| — having or using a single hand

one-horse |ˌwʌn ˈhɔːrs| — small and remote and insignificant

one-legged |ˈwʌnˈleɡd| — Having only one leg.

one-liner |ˌwʌnˈlaɪnər| — a one-line joke

one-man |ˈwən ˌman| — designed for or restricted to a single person

one-off |wʌnˈɒf| — a happening that occurs only once and is not repeated

one-on-one |ˈwʌn ɔːn ˈwʌn| — (of two persons) in direct encounter

one-party  — determined by a single party

one-piece |ˈwʌnˈpiːs| — (of clothing) consisting of or fashioned in a single whole piece

one-quarter  — one of four equal parts

one-sided |ˌwʌnˈsaɪdɪd| — favoring one person or side over another

one-sixth  — one part in six equal parts

one-stop |ˈwʌnstɒp| — (management) Describing a service operation at which all of a customer's needs may be satisfied without visiting another.

one-tenth  — a tenth part; one part in ten equal parts

one-third  — one of three equal parts of a divisible whole

one-time |ˈwʌntaɪm| — belonging to some prior time

one-to-one |wʌntəˈwʌn| — used of relations such that each member of one set is associated with one member of a second set

one-track |ˈwʌnˈtræk| — (of a railway) having a single set of lines so that trains can run only in one direction at a time; single-track

one-up  — (idiomatic) To outdo, to do something slightly better than a competitor's prior effort.

one-upmanship |wʌn ˈʌpmənʃɪp| — the practice of keeping one jump ahead of a friend or competitor

one-way |ˌwʌnˈweɪ| — moving or permitting movement in one direction only

one-year |ˈwəˈnjɪr| — completing its life cycle within a year

onefold |ˈwʌnfəʊld| — Constituting or being indicative of a single aspect or theme.

oneness |ˈwʌnnəs| — the quality of being united into one

oner |ˈwənər| — (informal) An extraordinary individual.

onerous |ˈɑːnərəs| — not easily borne; wearing

onerously |ˈəʊnərəslɪ| — in an onerous manner

oneself |wʌnˈself| — The person, previously mentioned.

onetime |ˈwʌntaɪm| — belonging to some prior time

onfall |ˈɑːnfɑːl| — A falling on or upon; an attack, onset, or assault.

ongoing |ˈɑːnɡəʊɪŋ| — currently happening

ongoings |ˈɒnˌɡəʊɪŋz| — plural form of ongoing

onhanger |ˈɒnˌhæŋə| — That which, or one who, hangs on to, relies on, or is dependent upon another; a dependant.

onion |ˈʌnjən| — the bulb of an onion plant

onionskin |ˈʌnjənˌskɪn| — a thin strong lightweight translucent paper used especially for making carbon copies

oniony |ˈʌnjənɪ| — Resembling an onion or onions, especially in terms of smell.

online |ˌɑːnˈlaɪn| — being in progress now

onlooker |ˈɑːnlʊkər| — someone who looks on

onlooking  — That is looking on; watching, observing.

only |ˈəʊnlɪ| — and nothing more

onomastic |ˌɑːnəˈmæstɪk| — of or related to onomastics

onomatopoeia |ˌɑːnəˌmætəˈpiːə| — using words that imitate the sound they denote

onomatopoeic |ˌɑːnəˌmætəˈpiːɪk| — of or relating to or characterized by onomatopoeia

onomatopoeical |ˌɒnəʊmətəʊˈpiːɪkəl| — (of words) formed in imitation of a natural sound

onrush |ˈɑːnrʌʃ| — (military) an offensive against an enemy (using weapons)

onset |ˈɑːnset| — the beginning or early stages

onside |ˌɑːnˈsaɪd| — not offside; being within the prescribed area of play

onslaught |ˈɑːnslɔːt| — a sudden and severe onset of trouble

onstage |ˌɑːnˈsteɪdʒ| — situated or taking place on the area of a stage visible to the audience

onto |ˈɑːntə| — Upon; on top of.

ontogenesis |ˌɑːntəˈdʒenɪsɪs| — (biology) the process of an individual organism growing organically; a purely biological unfolding of events involved in an organism changing gradually from a simple to a more complex level

ontogeny |anˈtɑːdʒənɪ| — (biology) the process of an individual organism growing organically; a purely biological unfolding of events involved in an organism changing gradually from a simple to a more complex level

ontological |ˌɑːntəˈlɑːdʒɪkl| — of or relating to ontology

ontology |ɑːnˈtɑːlədʒɪ| — (computer science) a rigorous and exhaustive organization of some knowledge domain that is usually hierarchical and contains all the relevant entities and their relations

onus |ˈəʊnəs| — an onerous or difficult concern

onward |ˈɑːnwərd| — forward in time or order or degree

onwards |ˈɑːnwərdz| — in a forward direction

onyx |ˈɑːnɪks| — a chalcedony with alternating black and white bands; used in making cameos