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OBSIDIAN: Dark volcanic glass, usually of felsic composition.

OFFSHORE: The marine zone extending from the breaker zone to the edge of the continental shelf.

OIL RESERVOIR: A bed of permeable and porous rock that contains commercially producible oil.

OIL SHALE: A dark-colored shale that contains organic material and that can be crushed and heated to liberate oil.

OIL TRAP: A tectonic or sedimentary structure that impedes upward movement of oil or gas and allows it to collect beneath the barrier.

OPHIOLITE SUITE: An assemblage of mafic and ultramafic igneous rocks with deep-sea sediments found on land, believed to be associated with divergent plate boundaries and the seafloor environment.

ORE DEPOSIT: A sedimentary, igneous, or metamorphic rock containing minerals, commonly metallic oxides or silicates, that can be commercially mined.

ORGANIC SEDIMENT, ROCK: A sediment or sedimentary rock consisting entirely or in part of organic carbon-rich deposits formed by the decay of once-living material after burial. Includes coal and organic carbon-rich shales.

ORIGINAL HORIZONTALITY, PRINCIPLE OF: The proposition that all sedimentary bedding is horizontal at the time of deposition.

OROGENIC BELT: A linear region that has been subjected to folding and other deformation in a mountain-building episode.

OROGENY: The tectonic process in which large areas are folded, thrust-faulted, metamorphosed, and subjected to plutonism. The cycle ends with uplift and the formation of mountains.

OUTCROP: A segment of bedrock exposed to the atmosphere.

OUTWASH: A sediment deposited by meltwater streams emanating from a glacier.

OVERTURNED FOLD: A fold in which a limb has tilted past vertical so that the older strata are uppermost.

OXBOW LAKE: A long, broad, crescent-shaped lake formed when a stream abandons a meander and takes a new course.

OXIDATION: A chemical reaction in which electrons are lost from an atom and its charge becomes more positive; a chemical combination of an element with oxygen.

OZONE: A molecule (O₃) that absorbs ultraviolet radiation in the stratosphere but creates smog when it forms near Earth's surface.