- oidiumA fragile spore produced by some fungi.
- ooeciumAn ovicell of a bryozoan.
- oogoneAn oogonium.
- oolithA spherical granule of which oolite is composed, formed by concentric accretion of thin layers of a mineral (usually calcium carbonate (limestone) but also others such as dolomite and silica) around a core; an ooid.
- oostegiteOne of the plates which, in some crustaceans, enclose a cavity in which the eggs are hatched.
- panoisticProducing ova only, and not yolk cells.
- spermatoonA spermatid.