China 
Africa 
- Odynophagia
- Oedometer tests
- Oersted
- Off
- Off-axis illumination
- Off-axis optical system
- Off-hook
- Off-line programming
- Off-road vehicles
- Off-site construction
- Off-target edits
- Off-the-grid
- Office Open XML
- Office automation
- Office chair
- Official statistics
- Offline learning
- Offset
- Offshore
- Offshore construction
- Offshore drilling
- Offshore geotechnical engineering
- Offshore wind farms
- Offshore wind power
- Offshoring
- Ogive
- Ohm
- Ohm's law
- Ohmic contact
- Ohmmeter
- Oil
- Oil additives
- Oil analysis
- Oil bath
- Oil burners
- Oil can
- Oil cooler
- Oil cooling
- Oil dispersants
- Oil equivalent
- Oil field
- Oil filter
- Oil heater
- Oil immersion
- Oil mist
- Oil paint
- Oil platforms
- Oil press
- Oil pressure
- Oil production
- Oil pump
- Oil red O
- Oil refinery
- Oil rigs
- Oil sands
- Oil seeps
- Oil shale
- Oil shale gas
- Oil shale reserves
- Oil sludge
- Oil spills
- Oil terminal
- Oil well
- Oil-based mud
- Oil-fired
- Oil-water contact
- Okara
- Okinawa Trough
- Okumura model
- Old age
- Oleanane
- Oleanolic acid
- Olecranon
- Olecranon fossa
- Olefin metathesis
- Oleic acid
- Oleochemicals
- Oleophobic
- Oleoresin
- Oleum
- Oleuropein
- Oleylamine
- Olfactometer
- Olfactory
- Olfactory bulb
- Olfactory ensheathing cells
- Olfactory epithelium
- Olfactory mucosa
- Olfactory nerve
- Olfactory receptor
- Olfactory system
- Olfactory tract
- Oligoclase
- Oligodendrocytes
- Oligodynamic effect
- Oligohydramnios
- Oligomers
- Oligomycin
- Oligonucleotides
- Oligopeptides
- Oligosaccharides
- Oligotrophic
- Oligotrophs
- Oliguria
- Olivine
- Olympic Dam
- Omega-3 fatty acids
- Omentum
- Omics
- Omitted-variable bias
- Ommatidia
- Omnibus test
- Omnichannel
- Omnidirectional antenna
- Omnidirectional camera
- Omnidirectional treadmill
- Omphacite
- On-orbit satellite servicing
- On-water reactions
- Oncogenes
- Oncology
- Oncolytic adenovirus
- Oncolytic viruses
- Oncosis
- Oncostatin m
- Oncotic pressure
- One-child policy
- One-class classification
- One-dimensional
- One-dimensional space
- One-factor-at-a-time method
- One-forms
- One-hot
- One-point compactification
- One-pot synthesis
- One-shot learning
- One-sided
- One-tailed test
- One-third octave
- One-time pad
- One-to-many
- One-way functions
- One-way mirror
- One-way wave equation
- Oniom
- Onlap
- Online Mendelian Inheritance in Man
- Online algorithms
- Online analytical processing
- Online chat