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oasis (n) – 1 : A fertile place in the desert, due to the presence of water  2 : Something that provides refuge, relief, or pleasant contrast

observation (n) – The act of noting and recording facts and events

obsolete (adj) – No longer in use, out of fashion

oceanic (adj) – Of, living in, or produced by the ocean

oceanic basin (n) – A great hollow in the Earth’s surface, filled by an ocean

oceanic ridge (n) – One of any number of extensive underwater mountain chains which are found in the Earth’s oceans and mark the boundaries of many of the Earth’s continental plates

ocelot (n) – A variety of North and South American wild cat; especially a large species with a yellow or gray coat and black spots

octave (n) – In musical terms, the eighth tone of an ascending or descending diatonic (major or minor) scale, or the interval created between one tone and another that is seven degrees in distance from the first in a diatonic scale

offshore (adj) – Far from shore

offspring (n) – Descendent, child, progeny

Oligocene (n) – The third part of the Tertiary Period (of the Cenozoic Era); between the Eocene and Miocene

ominous (adj) – Menacing; threatening

omnivore (n) – An organism that eats animals and plants

omnivorous (adj) – Both plant and meat-eating

on call (adj) – Available for service when called

ontogenetic (adj) – Having to do with the life cycle of a singe organism

opaque (adj) – A substance or material through which light cannot be seen; impenetrable by light; neither transparent nor translucent

open-door policy (n) – An immigration policy allowing (relatively) unrestricted admission to a country for all

operculum (n) – Body covering for gills

oppressive (adj) – Causing discomfort; hard to put up with

optimal (adj) – Most favorable or desireable

order (n) – The fourth largest categorical level of biological taxonomy, ranking below kingdom, phylum and class, and above family, genus and species

organic (adj) – 1 : Derived from living organisms 2 : A crop grown only with animal or vegetable fertilizers, such as manure, compost, bone meal, etc.

organic garden (n) – A garden grown only with the help of animal and vegetable fertilizer (no chemicals), including manure, bone meal, compost, etc.

organism (n) – Any living thing (plant, animal, fungus, etc.)

original sin (n) – The Christian belief that the sin of disobedience carried out by Adam and Eve is transmitted as guilt in all mankind

osprey (n) – A large, black-and-white, diving bird of prey belonging to a family of just one species and feeding mainly on fish; ospreys were once severely endangered but now their populations are coming back due to preservation efforts

outboard (n) – A portable motor with a propeller, mounted on the outside of the hull at a boat’s stern; a boat with an outboard motor

out-island (n) – A remote or outlying island within an island grouping or chain

outback (n) – The sparsely-settled, arid, inland region of Australia

overarching (adj) – Extending over or throughout

overharvest (v) – To harvest a particular species population beyond the point at which it can naturally recover in size

oxygenation (n) – A process that treats, combines, or infuses with oxygen

oxymoron (n) – A rhetorical figure in which incongruous or contradictory terms are combined

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