400+ Words Starting with O – Complete English List

Words starting with O are English terms that begin with the letter “O,” often used to express openness, order, and originality. These words appear in everyday language, studies, and professional writing.

In this guide, you’ll explore a wide range of Words Starting with O having clear meanings and examples to help improve your vocabulary and communication skills.

List of Words Starting with O

• Orange
• Onion
• Octopus
• Oven
• Ocean
• Olive
• OK
• Obj
• Ostrich
• Office
• Omelet
• Overcoat
• Orchard
• Oak Tree
• Oilcan
• Org
• Op
• Obs
• Octagon
• Observation
• Obligation
• Organization
• Opportunity
• Orientation
• Opposition

Grammar Words Starting with O

Nouns

• Ocean
• Opinion
• Office
• Object
• Occasion
• Outcome
• Owner
• Operation
• Officer

Verbs

• Observe
• Obtain
• Offer
• Operate
• Overcome
• Oversee
• Occupy
• Organize
• Offend

Adjectives

• Open
• Ordinary
• Obvious
• Official
• Organic
• Optimistic
• Obedient
• Outgoing
• Original

Adverbs

• Often
• Outwardly
• Overly
• Oddly
• Openly
• Officially
• Outright
• Once
• Otherwise

Collective Nouns

• Orchestra
• Outfit
• Order
• Organization
• Opposition
• Office Staff
• Ownership Group
• Outfield
• Operative Team

Scientific Words Starting with O

Orbit: A path one object takes around another in space.

• Earth moves in an orbit around the Sun.

Osmosis: The movement of water through a thin wall.

• Osmosis happens in cells to keep balance.

Oxidation: A process where a substance mixes with oxygen.

• Rust forms through oxidation of metal.

Optics: The study of light and vision.

• Optics helps design lenses and microscopes.

Ozone: A gas layer that protects Earth from the Sun’s rays.

• Ozone filters harmful ultraviolet light.

Organism: Any living thing, big or small.

• A plant is an example of a living organism.

Ovary: The part of a plant or animal that makes eggs.

• Flowers have ovaries that form seeds.

Oscillation: A back-and-forth motion that repeats.

• A pendulum moves in regular oscillation.

Oxygen: A gas needed by most living things to breathe.

• Humans inhale oxygen for energy.

Words Starting with O By Length

Two-Letter Words

• Of
• On
• Or
• Ox
• Oh
• Ow
• Oz
• Om

Three-Letter Words

• Oak
• Oar
• Oat
• Old
• One
• Orb
• Ore
• Owl
• Odd

Four-Letter Words

• Once
• Only
• Open
• Oven
• Over
• Omit
• Oath
• Obey
• Oily

Five-Letter Words

• Ocean
• Order
• Outer
• Other
• Olive
• Orbit
• Onion
• Offer
• Organ

Ending with E

• Olive
• Oppose
• Observe
• Obscure
• Overcome
• Offense
• Outline
• Opposite
• Operate

Double O Words

• Spoon
• Bloom
• Roomy
• Floor
• Brood
• Smooth
• Blood
• Scoot
• School

Historical Words

• Odyssey: A long adventurous journey or epic tale.
• Oligarchy: A government run by a small group of people.
• Ordinance: A local law or rule issued by authority.
• Oracle: A person believed to speak messages from gods.
• Obelisk: A tall stone monument from ancient Egypt.
• Oratory: The art of public speaking.
• Outpost: A small military camp away from the main base.
• Overlord: A ruler with great power over others.
• Occupation: Control of a place by foreign forces.

Topic-Based Words Starting with O

Animal Names

• Octopus
• Ocelot
• Owl
• Ox
• Otter
• Opossum
• Orca
• Ostrich
• Okapi

Country Names

• Oman
• Oceania
• Ottoman Empire
• Old Prussia
• Outer Mongolia
• Orissa
• Oaxaca
• Ossetia
• Ontario

Food Items

• Orange
• Omelet
• Oats
• Olive
• Onion
• Okra
• Oyster
• Oregano
• Oilcake

Plants and Trees

• Oak
• Olive Tree
• Orchid
• Oat Grass
• Oxeye Daisy
• Onion Plant
• Orange Tree
• Oleander
• Osmanthus

Technology Words Starting with O

• Operating System: The main software that manages computer hardware and programs.
• Online Network: A group of connected devices that share data through the internet.
• Output Device: Equipment that shows or delivers processed computer information.
• Optical Fiber: A thin glass strand used to send data as light signals.
• Object Code: The translated version of program code that a computer can run.
• Overclock: To make a computer’s processor run faster than its normal speed.
• Operator Mode: A system setting that allows advanced control or management.
• Onboard Memory: Built-in storage used by a device to hold data or files.
• Optimization Tool: Software used to improve system speed and performance.

City Names

• Oslo
• Ottawa
• Oxford
• Orlando
• Odessa
• Osaka
• Omaha
• Oakland
• Oporto

Household Items

• Oven
• Office Chair
• Outlet
• Oil Lamp
• Organizer
• Ottoman
• Opener
• Ornament
• Overhead Light

Job Titles

• Operator
• Officer
• Optician
• Organizer
• Observer
• Operator Assistant
• Orthodontist
• Oncologist
• Outreach Officer

Sports Words

• Offense
• Official
• Outfield
• Overtime
• Opponent
• Outrun
• Outscore
• Offside
• Overarm

School Vocabulary

• Oral Test
• Office
• Outline
• Objective
• Outcome
• Orientation
• Observation
• Option
• Organizer

Advanced Words Starting with O

• Obfuscate: To make something confusing or unclear intentionally.
• Obsequious: Excessively eager to please or obey someone.
• Obstreperous: Noisy, difficult to control, or unruly behavior.
• Ostentatious: Showy, designed to attract attention or impress.
• Omnipotent: Having unlimited power or authority over everything.
• Omniscient: Knowing everything, having complete or infinite knowledge.
• Onomatopoeia: Words that imitate sounds naturally or effectively.
• Ostensible: Appearing to be true but not real.
• Obdurate: Stubbornly refusing to change one’s mind.
• Oscillate: To move back and forth continuously or rhythmically.

Prefix Words Starting with O

Prefix	Example 1	Example 2	Example 3
Omni-	Omnipresent	Omniscient	Omnipotent
Over-	Overcome	Overload	Oversee
Out-	Outrun	Outshine	Outgrow
Ob-	Obstruct	Obtain	Observe
Off-	Offset	Offload	Offshoot
Opti-	Optimize	Optical	Optimism
Ortho-	Orthodontist	Orthopedic	Orthodox
Octa-	Octagon	Octave	Octopus
Osteo-	Osteology	Osteoporosis	Osteocyte

Positive Words Starting with O

• Open-minded
• Outstanding
• Optimistic
• Original
• Obedient
• Orderly
• Oblique
• Officious
• Overcast
• Ornate
• Outstretched
• Obvious
• Open-ended
• Ostentatious
• Opulent
• Oracular

Suffix Words Starting with O

Suffix	Example 1	Example 2	Example 3
-ous	Obvious	Odorous	Ominous
-tion	Operation	Observation	Obligation
-al	Official	Original	Optional
-ness	Openness	Orderliness	Oddness
-ist	Optimist	Organist	Orthopedist
-ism	Optimism	Organism	Opportunism
-ive	Objective	Offensive	Operative
-ary	Ordinary	Obituary	Oculary
-ent	Occurrent	Opponent	Omniscient

Words Starting with O For Kids

• Open
• One
• Old
• Out
• Off
• Owl
• Orange
• Over
• Office
• Object
• Option
• Order
• Offer
• Other
• Organ
• Owner

Medical Words Starting with O

Obesity: Having too much body fat.

• Regular exercise helps prevent obesity.

Orthopedic: Related to bones or muscles.

• Orthopedic doctors treat broken bones.

Oncology: The medical study of cancer.

• Oncology focuses on diagnosing and treating tumors.

Ophthalmology: The study of the eyes.

• Ophthalmology helps treat vision problems.

Otology: The branch of medicine about ears.

• Otology deals with hearing loss and infections.

Obstetrics: Care of women during pregnancy and birth.

• Obstetrics ensures healthy delivery.

Osteopathy: A treatment using gentle bone movement.

• Osteopathy helps ease back pain naturally.

Odontology: The scientific study of teeth.

• Odontology supports dental health care.

Organ Transplant: Moving an organ from one body to another.

• Organ transplant can save many lives.

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