- Ochre
- Octagon
- Octahedron
- Octane rating
- Octet rule
- Ocular dominance
- Odor
- Oersted
- Off-the-grid
- Offshore construction
- Offshore wind power
- Ohm
- Ohm's law
- Oil analysis
- Oil bath
- Oil filter
- Oil platforms
- Oil refinery
- Oil terminal
- Oil well
- Okinawa Trough
- Olefin metathesis
- Oligoclase
- Oligomers
- Oligotrophs
- Olivine
- Oncotic pressure
- Online refuelling
- Ooids
- Oolite
- Opacity
- Opal
- Open hearth furnace
- Open-channel flow
- Open-pit mining
- Open-source hardware
- Operating temperature
- Ophiolites
- Opinion polls
- Optical coating
- Optical fiber
- Optical lattices
- Optical modulators
- Optical radiation
- Optical rotation
- Optical spectrometer
- Optical windows
- Optoelectronics
- Opus spicatum
- Oral administration
- Oral and maxillofacial surgery
- Orbit
- Orbital decay
- Orbital hybridization
- Orbital inclination
- Orbital period
- Orbital plane
- Order of magnitude
- Orders of magnitude
- Organ
- Organelles
- Organic acids
- Organic compounds
- Organic electronics
- Organic laser
- Organic matter
- Organic peroxides
- Organic rankine cycle
- Organic semiconductors
- Organic solar cells
- Organic sulfide
- Organic synthesis
- Organolithium reagents
- Organometallic chemistry
- Organophosphate
- Orifice plates
- Origin
- Ornithopters
- Orthoclase
- Orthogonality
- Orthorhombic crystal system
- Orthosilicate
- Orthosilicic acid
- Orthostatic hypotension
- Orthotics
- Orthotropic material
- Oscillation
- Osmium
- Osmoregulation
- Osmotic coefficient
- Osmotic pressure
- Osseointegration
- Osteoarthritis
- Osteocalcin
- Osteoclasts
- Osteomalacia
- Osteopontin
- Osteoporosis
- Ostwald ripening
- Otavite
- Outboard motors
- Outgassing
- Ovarian cancer
- Overburden
- Overconsumption
- Overdiagnosis
- Overhead crane
- Overhead lines
- Oxalate
- Oxaliplatin
- Oxidation state
- Oxide minerals
- Oxidizing agents
- Oximes
- Oxonium ions
- Oxygen difluoride
- Oxygen evolution
- Oxygen saturation
- Oxygen scavengers
- Oxygen sensor
- Oxygen therapy
- Oxygen toxicity
- Oxygen-16
- Oxygen-18
- Oxygenates
- Oxyhydrogen
- Oyster
- Ozone
- Ozone cracking
- Ozone layer
- Ozone-depleting
- Ozonides
- Ozonolysis
- P waves
- P-n diode
- P-n junction
- PCA
- PCB
- PCR
- PEL
- PET
- PFBR
- PHWR
- PLD
- PPE
- PREN
- PSK
- PUE
- PWR
- Packed bed