140 Science Words That Start With O (With Meanings & Examples)

Learning new words helps you understand science better. Some of the most important terms in science begin with the letter O, and they appear in every subject from biology and chemistry to astronomy and mathematics. This guide will give you a complete list of 140 Science Words That Start With O, along with their meanings and examples. You’ll see how these terms are used in real science, from the orbit of planets to osmosis in plants.

Understanding these Science Words That Start With O will not only make learning easier but also improve your vocabulary in school and professional studies. Many of these terms show up in textbooks, research papers, and even daily conversations about health, the environment, or technology. To make things simple, each section explains the words clearly, with tables showing their meanings and example sentences.

Biology Science Words Starting With O

Word	Meaning	Example
Ovary	Female reproductive organs in plants and animals that produce eggs.	The flower’s ovary develops into fruit.
Osmosis	Movement of water molecules through a membrane from low to high solute concentration.	Plants absorb water by osmosis.
Osteocyte	A bone cell that maintains bone tissue.	Osteocytes help keep bones strong.
Oxygenation	Process of adding oxygen to tissues or blood.	Oxygenation happens in the lungs.
Olfactory	Related to the sense of smell.	Dogs have a strong olfactory system.
Oncology	Study of tumors and cancer.	Oncology doctors treat cancer patients.
Oogenesis	Process of egg cell formation in females.	Oogenesis occurs in the ovaries.
Ovary follicle	Structure in the ovary where eggs develop.	The ovary follicle matures before ovulation.
Ornithology	Study of birds.	She studied ornithology at university.
Organism	Any living being such as plants, animals, or microbes.	A frog is a simple organism.

Chemistry Science Words Starting With O

Word	Meaning	Example
Oxidation	Loss of electrons during a chemical reaction.	Rusting is caused by oxidation.
Oxide	A compound of oxygen with another element.	Iron oxide is also called rust.
Organic compound	A chemical containing carbon and hydrogen.	Glucose is an organic compound.
Ozone	A molecule made of three oxygen atoms.	Ozone protects Earth from UV rays.
Osmium	A dense metallic element used in alloys.	Osmium is the densest metal.
Octane	Hydrocarbon used in fuel.	Octane rating shows fuel quality.
Oleic acid	A fatty acid found in oils.	Olive oil contains oleic acid.
Orbitals	Regions around atoms where electrons are found.	Electrons move in orbitals.
Oxalic acid	A strong organic acid found in plants.	Spinach contains oxalic acid.
Ortho position	Position of groups on a benzene ring.	Ortho-xylene has two methyl groups next to each other.

Physics Science Words Starting With O

Word	Meaning	Example
Optics	Study of light and vision.	Optics explains how lenses work.
Oscillation	Repeated back-and-forth movement.	A pendulum shows oscillation.
Ohm	Unit of electrical resistance.	The circuit had a resistance of 5 ohms.
Orbit	Path of a body around another in space.	The moon orbits Earth.
Optical fiber	Thin strands that transmit light.	The Internet uses optical fiber cables.
Overtone	Higher frequency sounds produced with a main note.	Guitars produce overtones.
Oblique wave	A wave that hits a surface at an angle.	Oblique waves occur in water.
Optical illusion	Image that tricks the eye.	The drawing created an optical illusion.
Ohmmeter	Device to measure resistance.	He tested the circuit with an ohmmeter.
Oscilloscope	Device for displaying electrical signals.	The scientist checked signals on the oscilloscope.

Earth Science and Astronomy Words Starting With O

Word	Meaning	Example
Oceanography	Study of oceans.	Oceanography helps us understand marine life.
Orbiting satellite	Objects revolving around Earth.	Weather forecasts use orbiting satellites.
Obsidian	Volcanic glass rock.	Obsidian forms when lava cools quickly.
Orogeny	Process of mountain building.	The Himalayas formed by orogeny.
Overburden	Soil and rock covering minerals.	Mining requires removing the overburden.
Outer core	Layer of Earth beneath the mantle.	The outer core is made of liquid iron.
Outcrop	Rock formation visible on Earth’s surface.	The cliff showed a rock outcrop.
Ozone layer	Region of Earth’s atmosphere with ozone.	The ozone layer blocks harmful UV rays.
Orbit period	Time taken to complete one orbit.	Earth’s orbit period is 365 days.
Orion	A constellation visible in the night sky.	Orion is easy to spot in winter.

Medical Science Words Starting With O

Word	Meaning	Example
Orthopedics	Branch of medicine dealing with bones.	He studied orthopedics to treat fractures.
Ophthalmology	Study of eye diseases.	Ophthalmology focuses on vision health.
Obesity	Excessive fat in the body.	Exercise reduces obesity risks.
Osteoporosis	Weakening of bones.	Old people often suffer from osteoporosis.
Oral cavity	Mouth and its structures.	The dentist checked her oral cavity.
Ointment	Medicinal cream applied on skin.	He used ointment for the wound.
Organ transplant	Replacement of a diseased organ.	A kidney transplant saved his life.
Oncogene	Gene that can cause cancer.	Oncogenes trigger abnormal cell growth.
Oxygen therapy	Treatment with extra oxygen.	The patient needed oxygen therapy.
Otology	Study of ear and hearing problems.	Otology deals with ear infections.

Mathematics Words Starting With O

Word	Meaning	Example
Obtuse angle	Angle greater than 90° but less than 180°.	A triangle can have an obtuse angle.
Odd numbers	Numbers not divisible by 2.	3, 5, and 7 are odd numbers.
Ordinal numbers	Numbers showing position.	First, second, and third are ordinal numbers.
Origin	Starting point of a graph.	The graph began at the origin.
Operation	A mathematical process like +, −, ×, ÷.	Addition is a simple operation.
Order of operations	Rules for solving expressions.	BODMAS shows order of operations.
Orthogonal lines	Lines at right angles to each other.	The x and y axes are orthogonal.
Octagon	A shape with eight sides.	A stop sign is an octagon.
Outlier	A value far away from others in data.	100 was an outlier in the test results.
Open interval	Range between two values not including endpoints.	(2, 5) is an open interval.

General and Easy Science Words Starting With O

Word	Meaning	Example
Observation	Careful watching of something.	Scientists make observations during experiments.
Objective lens	Main lens in a microscope.	The objective lens gave a clear view.
Output	Result from a process or machine.	The computer gave the correct output.
Organ	A group of tissues with a function.	The heart is an important organ.
Outer space	Area beyond Earth’s atmosphere.	Astronauts travel into outer space.
Oil	A natural substance from plants or petroleum.	Machines need oil to work smoothly.
Optimum	Best possible condition.	Plants grow at optimum temperature.
Order	Sequence or arrangement.	Numbers can be written in order.
Overlap	When two things cover the same area.	The two circles overlap.
Outreach	Sharing knowledge with the community.	Science outreach inspires students.

Astronomy Science Words Starting With O

Word	Meaning	Example
Orbit	The curved path of a planet or object around another body.	Earth moves in an orbit around the Sun.
Observatory	A building equipped with telescopes for studying space.	Astronomers watched stars from the observatory.
Occultation	When one celestial object hides another from view.	The Moon caused an occultation of a star.
Oort Cloud	A region of icy bodies surrounding the solar system.	Comets come from the Oort Cloud.
Opposition	When a planet is directly opposite the Sun in the sky.	Mars appeared bright during opposition.
Outgassing	The release of gases from a planet or moon.	Volcanic outgassing shaped early Earth’s atmosphere.
Outer Planets	The large planets beyond the asteroid belt.	Jupiter is one of the outer planets.
Occam’s Razor	Principle suggesting the simplest explanation is best.	Scientists use Occam’s Razor in theories.
Obliquity	The tilt of a planet’s axis.	Earth’s obliquity causes seasons.
Optical Telescope	A telescope using light to view objects.	They used an optical telescope to see Saturn.

Environmental Science Words Starting With O

Word	Meaning	Example
Organic Matter	Natural material from plants and animals.	Soil with organic matter grows better crops.
Oxidation	A chemical reaction with oxygen.	Rust forms from oxidation of iron.
Overfishing	Catching too many fish in the oceans.	Overfishing harms marine life.
Overpopulation	Too many organisms in one area.	Overpopulation stresses resources.
Oil Spill	Release of oil into the environment.	An oil spill damaged the sea.
Overgrazing	Animals eat plants faster than they grow back.	Overgrazing leads to desert land.
Ozone	A gas layer protecting Earth from UV rays.	The ozone layer shields us from radiation.
Organic Farming	Growing food without chemicals.	Organic farming improves soil health.
Ocean Acidification	Oceans become more acidic from CO₂.	Ocean acidification harms coral reefs.
Offshore Wind	Wind energy generated at sea.	Offshore wind farms produce electricity.

Technology Science Words Starting With O

Word	Meaning	Example
Operating System	Software managing a computer’s functions.	Windows is an operating system.
Optics	Study of light and lenses.	Cameras rely on optics.
Oscillator	A device producing repeated motion or signal.	A clock uses an oscillator.
Optoelectronics	Devices using both light and electricity.	LEDs are optoelectronics.
Output	The result of a machine or system.	The output was displayed on screen.
Overload	When a system takes more than it can handle.	The circuit tripped from overload.
Online Data	Information available on the internet.	Scientists share online data quickly.
Optimization	Making something as effective as possible.	Engineers work on process optimization.
Onboard Computer	Computer built inside a machine or vehicle.	A plane uses an onboard computer.
Optical Fiber	Glass strands transmitting data as light.	The Internet runs on optical fiber cables.

Medical Science Words Starting With O

Word	Meaning	Example
Ophthalmology	Study of eyes and vision.	He studied ophthalmology to treat eye diseases.
Orthopedics	Medical science of bones and muscles.	Orthopedics helps broken bones heal.
Oncology	Study and treatment of cancer.	She works in oncology at the hospital.
Obesity	Excess body fat causes health risks.	Exercise reduces obesity.
Osteoporosis	Weak bones due to low density.	Calcium helps prevent osteoporosis.
Oxygen Therapy	Medical use of oxygen for patients.	The patient received oxygen therapy.
Oral Health	Care for teeth and mouth.	Brushing improves oral health.
Otolaryngology	Study of ear, nose, and throat.	Otolaryngology doctors treat ear infections.
Ovarian Cyst	Fluid-filled sac in the ovary.	She was diagnosed with an ovarian cyst.
Osmosis (Medicine)	Water movement in cells affecting health.	Kidney dialysis works with osmosis.

Genetics Science Words Starting With O

Word	Meaning	Example
Offspring	Children or young organisms.	Rabbits produce many offspring.
Oncogene	A gene that can cause cancer.	Mutation activated an oncogene.
Operon	A group of genes working together.	Bacteria use an operon to control proteins.
Oogenesis	Formation of eggs in females.	Oogenesis occurs in the ovaries.
Ontogeny	Development of an organism.	Frog ontogeny shows stages of growth.
Ortholog	Genes in different species with the same role.	Humans and mice share orthologs.
Overdominance	When hybrids have stronger traits.	Corn shows overdominance in growth.
Ovum	Female reproductive cell (egg).	Fertilization joins sperm and ovum.
Oligonucleotide	Short DNA or RNA sequence.	Scientists use oligonucleotides in labs.
Outcrossing	Breeding outside close relatives.	Farmers use outcrossing for healthier crops.

Plant Science Words Starting With O

Word	Meaning	Example
Orchid	A flowering plant family.	The orchid blooms in spring.
Oxygenation	Adding oxygen to soil or water.	Roots need oxygenation for growth.
Outgrowth	A small branch or part growing out.	The tree had a new outgrowth.
Ovule	Part of a plant that develops into seed.	The ovule grows after pollination.
Ovary (Plant)	Holds ovules and becomes fruit.	Apple ovaries form apples.
Organogenesis	Development of organs in plants.	Organogenesis happens during embryo growth.
Overwatering	Giving too much water to plants.	Overwatering kills roots.
Oleoresin	Plant oil and resin mixture.	Pine trees produce oleoresin.
Oxygen Cycle	Natural cycle of oxygen in nature.	The oxygen cycle keeps air balanced.
Ornamental Plants	Plants grown for decoration.	Roses are ornamental plants.

General Science Words Starting With O

Word	Meaning	Example
Observation	Careful watching to gain knowledge.	Scientists use observation in experiments.
Outcome	The result of a process.	The experiment’s outcome was successful.
Objectivity	Being unbiased and factual.	Science requires objectivity.
Order	An arrangement or sequence.	The order of steps matters in labs.
Operation	A process or method.	The operation was done with care.
Organization	A structured system or group.	Cells show organization in tissues.
Origin	The starting point of something.	The origin of rivers is mountains.
Output Energy	Energy released from a system.	The output energy was heat.
Optimum	The best condition for something.	Plants grow at optimum temperature.
Overestimate	To guess higher than real value.	Students overestimated the height of the tree.

Conclusion

This complete guide gave you 140 Science Words That Start With O along with meanings and examples. From osmosis in biology to the Oort Cloud in astronomy, these terms explain how life, matter, and the universe work. Learning them improves your science vocabulary with O, making it easier to read textbooks, understand lessons, and even prepare for exams.

Science is a language of its own, and every new word you learn brings you closer to mastery Science Words That Start With O. Whether you are a student looking for easy Science Words That Start With O for kids, or an advanced learner studying detailed science vocabulary list O words, this collection gives you the knowledge you need Science Words That Start With O.