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- O-type starStellar classification
- OB starOB stars are hot, massive stars of spectral types O or early-type B that form in loosely o
- Oberon (moon)Moon of Uranus
- Oberth effectType of spacecraft maneuver
- Observable universeAll of space observable from the Earth at the present
- Observation arcTime between earliest and latest observations of a Solar System body
- Observational astronomyDivision of astronomy & physics
- Observational cosmologyStudy of the origin of the universe (structure and evolution)
- ObservatoryLocation used for observing terrestrial or celestial events
- Observatory of StrasbourgThe Observatory of Strasbourg is an astronomical observatory in Strasbourg, France.
- Occator (crater)Occator is an impact crater located on Ceres, the largest object in the main asteroid belt
- OccultationWhen an object is hidden behind another
- OctansConstellation in the southern celestial hemisphere, containing the south celestial pole
- Odd number theoremThe odd number theorem is a theorem in strong gravitational lensing which comes directly f
- OGLE-TR-56bOGLE-TR-56b is an extrasolar planet located approximately 1500 parsecs or 5000 light years
- Olga BotnerPhysicist
- Olin J. EggenAmerican astronomer
- Olympus MonsMartian volcano, highest point on Mars
- Omega CentauriGlobular cluster in the constellation Centaurus
- Omega NebulaEmission nebula in Sagittarius
- OmenFuture-predicting phenomenon
- Omicron2 Canis MajorisOmicron2 Canis Majoris (ο2 CMa, ο2 Canis Majoris) is a star in the constellation Canis Maj
- Ondřejov ObservatoryAstronomical observatory on the eastern outskirts of Prague, Czech Republic
- Oort cloudDistant planetesimals in the Solar System
- Open clusterType of star cluster
- Ophelia (moon)Ophelia is a moon of Uranus. It was discovered from the images taken by Voyager 2 on Janua
- OphiuchusConstellation straddling the celestial equator
- Ophiuchus SuperclusterOphiuchus Supercluster is a nearby galaxy supercluster in the constellation Ophiuchus. The
- Opposition (astronomy)Two objects on opposite sides of the celestial sphere
- Optical Gravitational Lensing ExperimentLong-term variability sky survey
- Optical resolutionAbility of an imaging system to resolve detail
- OrbitCurved path of an object around a point
- Orbit insertionSpaceflight operation
- Orbit of the MoonThe Moon's circuit around Earth
- Orbital eccentricityAmount by which an orbit deviates from a perfect circle
- Orbital elementsParameters that define a specific orbit
- Orbital inclinationAngle between a reference plane and the plane of an orbit
- Orbital maneuverMovement during spaceflight
- Orbital mechanicsField of classical mechanics concerned with the motion of spacecraft
- Orbital nodePoint where an orbit crosses a plane of reference to which it is inclined
- Orbital passAn orbital pass (or simply pass) is the period in which a spacecraft is above the local ho
- Orbital periodTime an astronomical object takes to complete one orbit around another object
- Orbital resonanceRegular and periodic mutual gravitational influence of orbiting bodies
- Orbital speedSpeed at which a body orbits around the barycenter of a system
- Orbital stabilityIn mathematical physics and the theory of partial differential equations, the solitary wav
- Orbital state vectorsIn astrodynamics and celestial dynamics, the orbital state vectors (sometimes state vector
- Orbital station-keepingMaintenance of a particular orbit
- Orcus (dwarf planet)Trans-Neptunian dwarf planet
- Ordinary chondriteThe ordinary chondrites (sometimes called the O chondrites) are a class of stony chondriti
- Origin of the MoonTheories explaining the formation of Earth's Moon
- Orion (constellation)Constellation straddling the celestial equator
- Orion ArmMinor spiral arm of the Milky Way galaxy; contains the Solar System
- Orion NebulaDiffuse nebula in the constellation Orion
- Orion OB1Hot giant star group
- Orion's BeltAsterism of three stars within the constellation of Orion
- Osculating orbitOrbital perturbations
- Oskar BacklundSwedish-Russian astronomer (1846–1916)
- Otto HeckmannGerman astronomer (1901–1983)
- Otto Struve TelescopeThe Otto Struve Telescope was the first major telescope to be built at McDonald Observator
- Outer spaceVoid between celestial bodies
- Outflow channelsLong, wide swathes of scoured ground on Mars
- OutgassingRelease of a gas that was inside of some material
- Oxia PlanumPlain on Mars
- Oxygen-burning processThe oxygen-burning process is a set of nuclear fusion reactions that take place in massive
