- octyne
- odorization
- oenanthic
- off structure
- off-color
- off-time
- offset royalty
- offset shot
- offshore reserves
- oil base
- oil bloom
- oil cavity
- oil chiller
- oil clarifier
- oil classification
- oil collector
- oil cup
- oil dilution
- oil discovery
- oil dregs
- oil extractor
- oil filler
- oil filtration
- oil guard
- oil handling
- oil heater
- oil hose
- oil inlet
- oil interceptor
- oil lagging
- oil land
- oil layer
- oil length
- oil loss
- oil number
- oil polish
- oil press
- oil putty
- oil reclaiming
- oil rectifier
- oil regeneration
- oil shortage
- oil splitting
- oil spring
- oil stain
- oil storage
- oil store
- oil string
- oil terminal
- oil trace
- oil vaporizer
- oil vessel
- oil visor
- oil waste
- oil well
- oil wick
- oil zone
- oil-flash
- oil-impregnated
- oil-quenched
- oil-resistant
- oil-tanning
- oildom
- oiled leather
- oily materials
- ointment
- olefiant gas
- olefinic
- oleo oil
- oleophilic
- oleoresin
- oligeomerization
- oligomitic conglomerate
- olivine rock
- one diaphragm
- one-electron
- oil reserves
- oil resisting
- oil resources
- oil ring
- oil rock
- oil saturation
- oil saver
- oil seal
- oil still
- oil substitute
- oil switch
- oil vapor
- oil viscosity
- oil wiper
- oil-fired
- oilfield
- oiliness additive
- oilseed
- oily
- oily moisture
- old sand
- older tertiary
- oleaginous
- olefinic content