Ostraco. Denoting an oyster shell.

Otic. Of or relating to the ear.

Ovary. The reproductive organ in female animals in which eggs (ova) are produced. In most vertebrates there are two ovaries (in some fish the ovaries fuse together to form a single structure and in birds the left ovary only is functional.

They produce steroid hormones (see oestrogen; progesterone). In mammals each ovary is situated close to the opening of a fallopian tube; it contains numerous follicles in which the eggs develop and from which they are released in a regular cycle (see Graaffin follicle; menstrual cycle oogensis; ovaulation.
(2) The hollow base of the carpel of a flower, containing one or move ovules. After fertilization, the ovary wall develops into the fruit enclosing the seeds. In some species, the carpels are fused together to form a complex ovary.

Ovotertis. Organs of some hermaphrodite animals, e.g. snail, which functions both as ovary and testis.

Ovum (egg cell).
(1) (In zoology) The mature reproductive the ovary (see gamete) of female animals, which is produced by the ovary (see orgensis). It is spherical, has a nucleus, is covered with a vitelline membrane, and is not mobile.
(2) (in botany) The Oosphere of plants.

Oxidoreductase. One of a group of enzymes.

Ovoviviparous. Having embryos which develop within the maternal organism, from which they may derive nutriment, though they are separated from it by the persistence through most or all of development, of egg-membranes. e.g. many insects, snails, fish, lizards, and snakes.

Oxyhaemoglobin. Haemoglobin that is oxygenated.

Oxytocin. Hormone secreted from posterior lobe of the pituitary. A peptide of nine amino-acid residues, in chemical structure very similar to antidiuretic hormone. Synthesized in certain nerve cells of hypothalamus and conveyed by their axons to posterior pituitary, where released into blood. In female mammals, produces contraction of uterine muscles (especially at birth) and ejection of milk. Function in male mammals and non-mammalian vertebrates unclear.