oncomiracidium:

the ciliated larva of Monogenea that hatches from the egg.

oncosphere:

the six-hooked embryo that develops within the egg of Cestoda.

one-host tick:

a tick that attaches to one host as a larva, and stays on that host until development is complete and mating has occurred.

oocyst:

a cyst containing sporozoites in the Apicomplexa.

ookinete:

the motile zygote of malarial organisms.

ootype:

the region of the female reproductive system in Platyhelminthes that is surrounded by Mehli's gland and receives ducts from the vitellaria and seminal receptacle.

operculum:

a lid-like region on an egg that allows the larva to escape.

opisthaptor:

posterior attachment organ on Monogenea.

opposite:

located lateral to one another.

ovarian ball:

fragments of ovary of Acanthocephala that float free in the body cavity until fertilization.

ovary:

organ of female reproductive system that produces ova.

oviduct:

a short duct that carries ova from the ovary to the ootype.

ovijector:

a muscular modification at the termination of the female reproductive tract of Nematoda that controls the release of eggs.

ovisac:

external appendage on female Crustacea in which eggs are brooded until they hatch.