| Obligate | essential, necessary, unable to exist in any other state or relationship, e. g. obligate parasite |
| Oceanic zone | the region of open waters beyond the continental shelf and lying over deep areas |
| Ocellus/ocelli | One of the 3 simple eyes of adult insects, located on or near the centre line of the dorsal surface of the head. |
| Odonata | dragonflies and damselflies |
| Odour trail | A chemical (pheromone) trail, on the surface or in the air or water, deposited by one animal for another animal to follow. As in mating trails and foraging trails. |
| Oligochaeta | earthworms, pot worms, etc. |
| Oligolectic | taking pollen and/or nectar from just a few species of flower |
| Ommatidium | One of the basic visual units of the insect compound eye. The ommatidia are bounded externally by the facets that together make up the glassy, rounded outer surface of the eye. |
| Omnivore | An animal that eats both plant and animal food |
| Omnivorous | Feeding on a diet of plant and animal food |
| Ontogeny | The course of growth and development of an individual to maturity |
| Onychophora | Velvet worms |
| Ootheca | plural = oothecae An insect egg case (see cockroach). It may have a thin covering as in cockroaches, or the eggs may be embedded in a fast-hardening foam, as in mantids. |
| Open circulatory system | An arrangement of internal transport where the blood bathes the organs directly. |
| Operculum | a calcified flap or plate which closes the opening of some gastropod shells |
| Ophiuroidea | Brittle stars, a Cass in the Echinodermata. |
| Opiliones | harvestmen |
| Opisthobranchia | a sub class of the Gastropoda |
| Opportunistic species | A species adapted to use variable, unpredictable or temporary environments. Usually they have a high dispersal ability, and high and rapid population growth. |
| Optimal | most favourable |
| Optimal foraging | a type of foraging behaviour that maximizes the ratio of energy intake to expenditure |
| Opithosoma | the posterior body (abdomen) in arachnids and pogonophorans |
| Oral | of the mouth |
| Orb web spider | a spider in the Araneidae family |
| Order | the major taxonomic division of a class |
| Organ | A structure of the body composed of several kinds of tissue that carries out specific tasks, e.g. heart |
| Organic | In chemistry -containing the element carbon. In nature - connected with living material. |
| Organelle | A membrane-bound body in a cell, e.g. mitochondria. An organelle performs a specific function. |
| Orthoptera | the Order of insects containing the grasshoppers, locusts, crickets, katydids, etc. |
| Osculum | the excurrent opening in sponges |
| Osmoregulation | the maintenance of a suitable internal salt/water concentration in a cell or body |
| Osmosis | the process whereby water passes across a membrane from the weaker to the stronger solution |
| Ossicles | the small, separate pieces of echinoderm skeleton |
| Ostia | a pore through which water enters the body (as in sponges), or through which blood enters the heart |
| Ostracoda | mussel shrimps, seed shrimps, a Class in the Crustacea phylum |
| Outbreeding | The mating or crossing of individuals that are either less closely related than average in the population, or from different populations. The opposite of inbreeding. |
| Ovary | the structure producing female gametes and reproductive hormones |
| Oviduct | In invertebrates, a tube passing from the ovary to the vagina |
| Oviger | the egg carrying leg in the pycnogonids |
| Oviparous | Reproducing by laying eggs |
| Oviposition | the act of depositing eggs |
| Ovipositor | The egg laying tube in some female insects. In some bees and wasps it has been modified to become a sting. |