ocean basin	A depression in the earth's crust between continents, comprised mainly of basalt; the containment vessel for an ocean.
Oceanic province	A pelagic division of the ocean, located beyond the continental shelf.
oceanography	The branch of science dealing with the physical and biological features of the sea.
offspring	The immediate descendant or descendants of a person, animal etc; progeny.
oil	Any of a number of viscious liquids with a sticky feel. They are usually flammable, insoluble in water, soluble in organic solvents, and are obtained from plants and animals, from mineral deposits and by extraction.
opportunistic species	Small species with a high reproduction potential, easily able to colonise azoic areas.
optimal growth conditions	The environmental or/and cultivation conditions under which a population can achieve the maximum survival and somatic growth rate.
organic	1. Concerning chemical compounds based on carbon chains or rings and also containing hydrogen with or without oxygen, nitrogen and other elements 2. Pertaining to or derived from living.
organic enrichment	The process of increasing or enhancing some constituent or property, particularly through the addition of certain materials.
osmoregulation	The process by which organisms maintain a stable solute concentration. The maintenance of osmotic pressure on each side of a semipermeable membrane, i.e. osmotic balance.
osmosis	Diffusion of a solvent through a semipermeable membrane from a region of low solute concentration to one of high solute concentration, thereby diluting the latter. Osmosis will continue until the concentrations of the two solutions are equalized.
over-exploitation (fisheries)	Exploitation of a fish stock to the extent that the population and catch goes into decline.
oxidation	That part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a different reactant must occur.
oxygen	A colourless odourless highly reactive gaseous element: the most abundant element in the earth's crust. Symbol: 0. Atomic number: 8.
oxygen consumption	The rate of use of oxygen (O2/mg/kg/hr) by the organisms in a holding unit. Increases with feeding/ swimiing activity and poor water quality conditions (e.g. high ammonia levels).
oxygenation	1. In aquaculture: the input of pure oxygen into the culture medium to enhance or supplement its oxygen content; this promotes lower water exchange rates in the system 2. In physiology: the oxygen transfer to blood cells (processes of uptake and release of haemoglobin bound oxygen).
ozone	A specific molecular (allotropic) state of oxygen. The ozone molecule contains three oxygen (molecular weight of 48.00 grams per mole). Ozone is the fourth most powerful oxidizing agent known (only F2, F2O, and O· are better) and is thought to have a mechanism of oxidation related to the following reaction: O3-> O2+ O·, where nascent oxygen produces a high-energy oxidation via free radical reaction.