| Opportunistic | | An arrangement of internal transport in which blood bathes the organs directly and there is no distinction between blood and interstitial fluid. 3 Words |
| OralCavity | | A unit of genetic function common in bacteria and phages, consisting of coordinately regulated clusters of genes with related functions. |
| Operon | | A specialized center of body function composed of several different types of tissues. |
| Osmolarity | | A measure of the tendency of a solution to take up water when separated from pure water by a selectively permeable membrane. 2 Words |
| Osmosis | | The diffusion of water across a selectively permeable membrane. |
| Osculum | | A group of organs that work together in performing vital body functions. 2 Words |
| Operator | | An arrangement of the fingers such that the thumb can touch the ventral surface of the fingertips on all four fingers. 2 Words |
| Osmoconformer | | The development of organ rudiments from the three germ layers. |
| OpposableThumb | | In prokaryotic DNA, a sequence of nucleotides near the start of an operon to which an active repressor can attach. |
| OsmoticPressure | | An animal that does not actively adjust to its internal osmolarity because it is isotonic with its environment. |
| Order | | In classification, the taxonomic category above family. |
| Organogenesis | | A large opening in a sponge that connects the spongocoel to the environment. |
| OrganicChemistry | | A heterotrophic animal that consumes both meat and plant material. |
| Omnivore | | The mouth of an animal. 2 Words |
| Osmoregulation | | Microorganisms that are normal residents of a host but can cause illness when the host's defenses are weakened by such factors such as poor nutrition or a recent bout with the flu. |
| OpenCirculatorySystem | | One of several formed bodies with specialized functions, suspended in the cytoplasm of eukaryotic cells. |
| Organ | | Bone-forming cells that deposit a matrix of collagen. |
| OrganSystem | | Solute concentration expressed as molarity. |
| Organelle | | The study of carbon compounds. 2 Words |
| Osteoblasts | | The control of water balance in organisms living in hypertonic, hypotonic, or terrestrial environments. |
