This list includes 49 Hard words that start with O, from “obambulate” to “oviparous”. They are mostly low-frequency, polysyllabic terms that come from technical, literary, or archaic registers. You can use them for vocabulary study, advanced writing, editing, and ESL practice.
[Hard words that start with O] are challenging, uncommon English words often found in literature, science, law, and specialized fields. Many of these terms trace back to Latin, Greek, or Old English and appear in older dictionaries and classic texts.
Below you’ll find the table with Etymology and Definition.
Etymology: Shows each word’s origin language, root elements, and an approximate date, so you can trace how meanings developed.
Definition: Gives a concise part-of-speech label plus a one-sentence meaning you can use for study or writing.
Hard words that start with O
| Word | Part of speech | Pronunciation | Etymology |
|---|---|---|---|
| obambulate | verb | (ob-AM-byoo-layt) | Latin *obambulare* ‘to walk up to’, from the 17th century. |
| obdurate | adjective | (OB-doo-rit) | Latin *obdurare* ‘to harden’, from the 15th century. |
| obeisance | noun | (oh-BAY-sunss) | Old French *obeissance* ‘obedience’, from the 14th century. |
| obfuscate | verb | (OB-fuh-skayt) | Latin *obfuscare* ‘to darken’, from the 16th century. |
| objurgate | verb | (ob-JUR-gayt) | Latin *objurgare* ‘to chide or rebuke’, from the 16th century. |
| oblation | noun | (oh-BLAY-shun) | Latin *oblatio* ‘an offering’, from the 14th century. |
| obloquy | noun | (OB-luh-kwee) | Latin *obloqui* ‘to speak against’, from the 15th century. |
| obnubilate | verb | (ob-NOO-bih-layt) | Latin *obnubilare* ‘to cover with clouds’, from the 16th century. |
| obsequious | adjective | (ob-SEE-kwee-us) | Latin *obsequiosus* ‘compliant’, from the 15th century. |
| obsequy | noun | (OB-suh-kwee) | Anglo-French *obsequie*, from Latin *exsequiae* ‘funeral rites’, 15th century. |
| obsolescent | adjective | (ob-suh-LESS-ent) | Latin *obsolescere* ‘to fall into disuse’, from the 18th century. |
| obstreperous | adjective | (ob-STREP-er-us) | Latin *obstreperus*, from *obstrepere* ‘to make a noise’, 16th century. |
| obtund | verb | (ob-TUND) | Latin *obtundere* ‘to beat against, blunt’, from the 15th century. |
| obturate | verb | (OB-too-rayt) | Latin *obturare* ‘to stop up’, from the 16th century. |
| obverse | noun | (OB-vurss) | Latin *obversus* ‘turned toward’, from the 17th century. |
| obviate | verb | (OB-vee-ayt) | Latin *obviare* ‘to prevent’, from the 16th century. |
| occlude | verb | (uh-KLOOD) | Latin *occludere* ‘to shut up’, from the 16th century. |
| occult | adjective | (uh-KULT) | Latin *occultus* ‘hidden, secret’, from the 16th century. |
| odalisque | noun | (OH-duh-lisk) | French, from Turkish *odalık* ‘chambermaid’, from the 17th century. |
| odontalgia | noun | (oh-don-TAL-jee-uh) | Greek *odon* ‘tooth’ + *algos* ‘pain’, from the 18th century. |
| odoriferous | adjective | (oh-duh-RIF-er-us) | Latin *odorifer* ‘fragrant’, from the 15th century. |
| officious | adjective | (uh-FISH-us) | Latin *officiosus* ‘dutiful’, from the 15th century. |
| oleaginous | adjective | (oh-lee-AJ-uh-nus) | Latin *oleaginus* ‘of the olive tree’, from the 15th century. |
| oligarchy | noun | (OL-i-gahr-kee) | Greek *oligarkhia* ‘rule by the few’, from the 16th century. |
| omnibus | adjective | (OM-ni-bus) | Latin *omnibus* ‘for all’, from the 19th century. |
| omnifarious | adjective | (om-ni-FAIR-ee-us) | Latin *omnifarius* ‘of all sorts’, from the 16th century. |
| onomasiology | noun | (oh-nuh-mah-see-OL-uh-jee) | Greek *onomasia* ‘naming’ + *logia* ‘study’, from the 19th century. |
| onomastics | noun | (on-uh-MAS-tiks) | Greek *onomastikos* ‘of or for a name’, from the 19th century. |
| ontology | noun | (on-TOL-uh-jee) | New Latin *ontologia*, from Greek *on* ‘being’, from the 18th century. |
| onerous | adjective | (OH-ner-us) | Old French *onereus*, from Latin *onus* ‘burden’, 14th century. |
| opalescent | adjective | (oh-puh-LESS-ent) | From the gem ‘opal’ + ‘-escent’, from the 19th century. |
| oppidan | noun | (OP-i-dun) | Latin *oppidanus* ‘of a town’, from the 16th century. |
| opprobrium | noun | (uh-PROH-bree-um) | Latin *opprobrare* ‘to reproach’, from the 17th century. |
| oppugn | verb | (uh-PYOON) | Latin *oppugnare* ‘to fight against’, from the 15th century. |
| opuscule | noun | (oh-PUS-kyool) | Latin *opusculum* ‘little work’, from the 17th century. |
| oracular | adjective | (aw-RAK-yuh-ler) | Latin *oraculum* ‘divine announcement’, from the 16th century. |
| orison | noun | (OR-i-zun) | Old French *oraisun* ‘prayer’, from the 13th century. |
| orotund | adjective | (OR-uh-tund) | Latin *ore rotundo* ‘with round mouth’, from the 18th century. |
| orthoepy | noun | (or-THOH-uh-pee) | Greek *orthoepeia* ‘correct speech’, from the 17th century. |
| orthography | noun | (or-THOG-ruh-fee) | Greek *orthographia* ‘correct writing’, from the 15th century. |
| oscitant | adjective | (OS-i-tuhnt) | Latin *oscitare* ‘to yawn’, from the 18th century. |
| osculate | verb | (OS-kyuh-layt) | Latin *osculari* ‘to kiss’, from the 17th century. |
| ossify | verb | (OS-uh-fahy) | French *ossifier*, from Latin *os* ‘bone’, from the 18th century. |
| ostensible | adjective | (o-STEN-suh-buhl) | French, from Latin *ostendere* ‘to show’, from the 18th century. |
| ostentatious | adjective | (os-ten-TAY-shus) | Latin *ostentatio* ‘display’, from the 16th century. |
| otiose | adjective | (OH-shee-ohs) | Latin *otiosus* ‘at leisure’, from the 18th century. |
| outré | adjective | (oo-TRAY) | French *outrer* ‘to overdo’, from the 18th century. |
| overweening | adjective | (oh-ver-WEEN-ing) | Middle English *overwenen* ‘to be arrogant’, 14th century. |
| oviparous | adjective | (oh-VIP-uh-rus) | Latin *ovum* ‘egg’ + *parere* ‘to bring forth’, from the 17th century. |
Descriptions
obambulate
A verb that means to walk about or wander aimlessly from place to place.
obdurate
An adjective describing someone who is stubbornly refusing to change their opinion or course of action.
obeisance
A noun for a gesture expressing deep respect, such as a bow or curtsy.
obfuscate
A verb meaning to render something obscure, unclear, or unintelligible, often deliberately.
objurgate
A verb that means to scold or rebuke someone severely for their wrongdoing.
oblation
A noun for the act of offering something, like a gift or sacrifice, to a god.
obloquy
A noun for strong public criticism or verbal abuse directed at a person or thing.
obnubilate
An obscure verb meaning to darken, becloud, or make something mentally foggy or confused.
obsequious
An adjective describing someone who is overly obedient or attentive in a servile way.
obsequy
A noun, usually used in the plural (obsequies), that refers to funeral rites or ceremonies.
obsolescent
An adjective describing something that is in the process of becoming outdated or no longer used.
obstreperous
An adjective for someone who is noisy, unruly, and difficult to control, especially a child.
obtund
A verb meaning to dull the sensitivity of something, such as a pain or a sense.
obturate
A technical verb meaning to block or close up an opening or passage.
obverse
A noun for the side of a coin or medal that bears the head or principal design.
obviate
A verb meaning to remove a need or difficulty, thereby preventing a potential problem.
occlude
A verb meaning to stop, close up, or obstruct an opening, passage, or sight.
occult
An adjective relating to mystical, supernatural, or magical powers, practices, or phenomena.
odalisque
A noun for a female slave or concubine in a harem, especially in the Turkish sultan’s palace.
odontalgia
A noun that is the technical medical term for a toothache.
odoriferous
An adjective meaning having or giving off a smell, which can be either pleasant or unpleasant.
officious
An adjective for someone who is annoyingly eager to give unwanted advice or help; meddlesome.
oleaginous
An adjective meaning oily or greasy, or describing a person with an unpleasantly smooth and ingratiating manner.
oligarchy
A noun for a form of government where power rests with a small number of elite people.
omnibus
An adjective describing a volume containing several works previously published separately.
omnifarious
An adjective meaning of all varieties, forms, or kinds.
onomasiology
A noun for the branch of linguistics that deals with the study of names and naming.
onomastics
A noun referring to the study of the history and origin of proper names, especially personal names.
ontology
A noun for the branch of metaphysics dealing with the nature of being and existence.
onerous
An adjective describing a task or responsibility that involves a great deal of effort and difficulty.
opalescent
An adjective describing something that exhibits a milky, iridescent luster like that of an opal.
oppidan
A noun for an inhabitant of a town, as distinct from a member of a university.
opprobrium
A noun for the public disgrace arising from shameful conduct or harsh criticism.
oppugn
A verb meaning to question the truth or validity of something; to dispute or call into question.
opuscule
A noun for a small or minor literary or musical work.
oracular
An adjective describing a person or statement as prophetic, authoritative, or difficult to interpret.
orison
A noun, now chiefly literary or archaic, that simply means a prayer.
orotund
An adjective describing a voice that is deep and resonant, or language that is pompous and pretentious.
orthoepy
A noun for the study of correct pronunciation in a language.
orthography
A noun for the conventional spelling system of a language.
oscitant
An adjective describing someone who is drowsy, inattentive, or lazy; characterized by yawning.
osculate
A formal or jocular verb meaning to kiss, or a technical term for when curves touch at a point.
ossify
A verb meaning to turn into bone, or to become rigid and inflexible in one’s habits or beliefs.
ostensible
An adjective used to describe something that appears to be true but is not necessarily so.
ostentatious
An adjective describing a pretentious and vulgar display of wealth and luxury designed to impress.
otiose
An adjective meaning serving no practical purpose or result; idle or indolent.
outré
An adjective borrowed from French that describes something as bizarre, unusual, or violating convention.
overweening
An adjective describing someone who shows excessive confidence or pride; arrogant.
oviparous
A biological adjective for animals, such as birds and reptiles, that produce young by means of eggs.