O

Obligate aerobe: An organism that requires oxygen for cellular respiration and cannot live without it.
Obligate anaerobe: An organism that cannot use oxygen and is poisoned by it.
Occam’s razor: The general principle that if all else is equal, the simplest explanation is best. It is applied when using parsimony methods during phylogenetic reconstruction.
Oceanic pelagic biome: Most of the ocean’s water far from shore, constantly mixed by ocean currents.
Oceanic zone: The region of water lying over deep areas beyond the continental shelf.
Okazaki fragments: Small fragments of DNA made during DNA replication of the lagging strand.
Olfactory receptor: Smell receptor.
Oligodendrocyte: A type of glial cell that forms insulating myelin sheaths around the axons of neurons in the central nervous system.
Oligonucleotides: A short piece of DNA, no more than about 20 base pairs long.
Oligosaccharin: A type of elicitor (molecule that induces a broad defense response in plants) that is derived from cellulose fragments released by cell wall damage.
Oligotrophic lake: A nutrient-poor, oxygen-rich clear, deep lake with few phytoplankton.
Ommatidium: One of the facets of the compound eye of arthropods and some polychaete worms.
Omnivorous: Feeding on both animal and vegetable substances.
Oncogene: A gene found in viruses or as part of the normal genome that is involved in triggering cancerous characteristics.
Oogenesis: The process in the ovary that results in the production of female gametes.
Oogonia: Ovary-specific stem cells. The plural of oogonium. Oogonia are formed by mitosis of primordial germ cells, and are precursor cells of primary oocytes.
Oomycete: A protist with flagellated cells, such as a water mold, white rust, or downy mildew, that acquired nutrition mainly as a decomposer or plant parasite.
Onychophoran: Member of a phylum of caterpillar-like animals. The living forms are terrestrial and are commonly referred to as velvet worms; the fossils are marine and were diverse in the Cambrian (phylum Onychophoran).
Open circulatory system: The circulatory system in which fluid called hemolymph bathes the tissues and organs directly and there is no distinction between the circulating fluid and the interstitial fluid.
Open reading frame (ORF): A section of a genome that contains the codons used to make a protein. In bacteria and archaea ORFs are usually found in contiguous stretches in the genome; in eukaryotes they are frequently interrupted by introns.
Operational gene: Genes involved in metabolic and other peripheral processes. They are meant to contrast with informational genes are thought to be more prone to lateral gene transfer than informational genes.
Operational taxonomic unit (OTU): Any entity (e.g. an organism, gene, species, or population) used for phylogenetic study.
Operator: In prokaryotic DNA, a sequence of nucleotides near the start of an operon to which an active repressor can attach.  The binding of the repressor prevents RNA polymerase from attaching to the promoter and transcribing the genes of the operon.
Operculum: In aquatic osteichthyans, a protective bony flap that covers and protects the gills.
Operon: A set of adjacent genes, common in bacteria and phages, whose transcription is regulated as a single unit.
Opisthokont: Member of a kingdom of eukaryotes including the metazoan, fungi, and choanoflagellates.
Opposable thumb: An arrangement of the fingers such that the thumb can touch the ventral surface of the fingertips of all four fingers.
Opsin: A membrane protein bonded to a light-absorbing pigment molecule.
Optic chiasm: The arrangement of the nerve tracts of the eye such that the visual sensations from the left visual field of both eyes are transmitted to the right side of the brain and the sensations from the right visual field of both eyes are transmitted to the left side of the brain.
Optimal feeding theory: The basis for analyzing behavior as a compromise of feeding costs versus feeding benefits.
Oral cavity:  The mouth of an animal.
Orbital: The three-dimensional space where an electron is found 90% of the time.
Order: In classification, the taxonomic category above family.
Organ: A specialized center of body function composed of several different types of tissue.
Organ identity genes: Plant homeotic genes that use positional information to determine which emerging leaves develop into which types of floral organs.
Organ of Corti: The actual hearing organ of the vertebrate ear, located in the floor of the cochlear canal in the inner ear; contains the receptor cells (hair cells) of the ear.
Organ system: A group of organs that work together in performing vital body functions.
Organelle: One of several formed bodies with specialized functions, suspended in the cytoplasm of eukaryotic cells.
Organic chemistry: The study of carbon compounds; organic compounds.
Organism:  Any life form considered an entity; animal, plant, fungi, bacteria, etc.
Organismal ecology: The branch of ecology concerned with the morphological, physiological, and behavioral ways in which individual organisms meet the challenges posed by their biotic and abiotic environments.
Organogenesis: The development of organ rudiments from the three germ layers.
Origin of replication: Site where the replication of a DNA molecule begins.
Orthogenesis: An inherit tendency for lineages to change in a particular direction.
Orthologous genes: Genes that are homologous (share a common ancestry) and have diverged from each other due to the separation of the species in which the genes are found (e.g. a-hemoglobin from humans and mice). Contrast with paralogous genes.
Osculum: A large opening in a sponge that connects the spongocoel to the environment.
Osmoconformer: An animal that does not actively adjust its internal osmolarity because it is isoosmotic with its environment.
Osmolarity: Solute concentration expressed as molarity.
Osmoregulation: How organisms regulate solute concentrations and balance the gain and loss of water.
Osmoregulator: An animal whose body fluids have a different osmolarity than the environment and that must either discharge  excess water if it lives in a hypoosmotic environment or take in water if it inhabits a hyperosmotic environment.
Osmosis: The diffusion of water across a selectively permeable membrane.
Osmotic potential: A component of water potential that is proportional to the number of dissolved solute molecules in a solution and measures the effect of solutes on the direction of water movement; also called  solute potential.
Osteichthyan: Member of a vertebrate subgroup with jaws and mostly bony skeleton.
Osteoblast: A bone-forming cell that deposits collagen.
Osteom: The repeating organizational unit forming the microscope structure  of hard mammalian bone.
Outcrossing: Mating with unrelated individuals.
Outer ear: One of three main regions of the ear in reptiles, birds, and mammals, made up of the auditory canal and, in many birds and mammals, the pinna.
Outgroup: An organism or gene from an evolutionary lineage that separated from those lineages being study prior to the existence of their common ancestor.
Out-of-Africa model: The hypothesis that modern humans evolved recently in Africa and spread from there, replacing archaic hominins. It contrasts with the multiregional model.
Oval window: In the vertebrate ear, a membrane-covered gap in the skull bone, through which sound waves pass from the middle ear to the inner ear.
Ovarian cycle: The cycle recurrence of the follicular phase, ovulation, and the luteal phase in the mammalian ovary; regulated by hormones.
Ovary: (1) In flowers, The portion of a carpel in which the egg-containing ovules develop. (2) In animals, The structure that produces female gametes and reproductive hormones.
Overdominance: Describes heterozygotes that have higher trait values (usually higher fitness) than either homozygote.
Overexploitation: Harvesting by humans of wild plants and animals at rates exceeding the ability of populations of those species to rebound.
Overnourishment: A diet that is chronically excessive in calories.
Oviduct: A tube passing from the ovary to the vagina in invertebrates or to the uterus in vertebrates.
Oviparous: Referring to a type of development in which young hatch from eggs that are retained in the mother’s uterus.
Ovulation: The release of an egg from ovaries. In humans, an ovarian follicle releases an egg during each uterine (menstrual) cycle.
Ovule: The female gamete in a flowering plant. It is analogous to the egg in animals.
Ovum: The female gamete; the haploid, unfertilized egg, which is usually a relatively large, nonmotile cell.
Oxic: Environment with detectable levels of free oxygen.
Oxidation: The loss of electrons from a substance involved in a redox reaction.
Oxidative phosphorylation: The production of ATP using energy derived from the redox reactions of an electron transport chain.
Oxidizing agent: The electron acceptor in a redox reaction.
Oxytocin: A hormone produced by the hypothalamus and released from the posterior pituitary. It induces contractions of the uterine muscles and causes the mammary glands to eject milk during nursing.