- oligohaline
- one-fluid
- onionskin paper
- onlap
- onset
- onshore
- onshore basis
- oolithic
- oolithic limestone
- oolithic structure
- oolithic texture
- opacifier
- opacite
- opal
- opalescent
- opaque
- opaque ice
- opdalite
- open cavity
- open cell
- open cup
- open cure
- open cut
- open end
- open mill
- open pit
- open pressure
- open structure
- opener
- opepe
- operating agreement
- operating efficiency
- operating engineer
- operating line
- operating loss
- operating plan
- operating pressure
- operating provision
- operation
- operational research
- operator
- ophitic texture
- opianic
- opianyl
- opium
- optic anisotropy
- optical activity
- optical brightener
- optical character
- optical glass
- optical index
- optical isomer
- optical plastics
- obeche
- obligated capital
- oblique bedding
- oblique illumination
- oblique lamination
- oblique section
- obscured
- obsequent valley
- observation
- observation period
- occlude
- occluded
- occlusion
- ochreous clay
- octahedral cleavage
- octane scale
- odometer
- odorize
- oenanthal
- offset oil
- offset printing
- offshore drilling
- oil absorption
- oil aeration
- oil bearing
- oil blending
- oil brand
- oil brush
- oil capacity
- oil column
- oil content
- oil corrosion
- oil decomposition
- oil delivery
- oil dispersion
- oil drag
- oil engineer
- oil equipment
- oil fogging
- oil fume
- oil gas
- oil genesis
- oil grain
- oil hardening
- oil immersion
- oil insoluble
- oil jack