Occupational Safety and Health Administration
OCS
Offset well
Offshore drilling
Ohm
Ohmic
Ohm-meter
Ohm's law
Oil-base mud
Oil-cut
Oil emulsion
Oil field
Oil in place
Oil mud
Oil patch
Oil shale
Oil stain
Oil-water contact
Oil-water interface
Oil wet
Oleophilic
Open hole
Openhole completion
Operator
OSHA
Osmosis
Outcrop
Overburden
Overlay
Overpressure
Overshot
Oxidation-reduction potential