An oospore is a sexually produced spore that develops from unequal gametangial copulation or markedly unequal (oogamous) gametic fusion. It is the characteristic sexually produced spore of the Oomycota, although oospores are also found in the Monoblepharidales (Chytridiomycota).
In the Oomycota, oospore development begins with the formation of one or more oospheres within the larger gametangium, the oogonium. After fertilization, i.e. the receipt of an antheridial nucleus by the oosphere, this lays down a thick wall and becomes the oospore.
The number of oospores per oogonium may vary, and this is an important taxonomic criterion. Meiotic nuclear divisions precede oosphere and antheridial maturation in the Oomycota and nuclear fusion follows fertilization, so that the oospore is diploid.
The oospore develops a thick outer wall and lays down food reserves, usually in the form of lipids. In the Peronosporales the outer wall of the oospore is surrounded by periplasm, the residual cytoplasm left in the oogonium after the oospheres have been cleaved out.
Oospores are sedentary (memnospores) and are important in survival rather than dispersal. They often require a period of maturation before germination can occur and may remain dormant for long periods.