An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and Oomycetes. In Oomycetes, oospores can also result from asexual reproduction, by apomixis.

The oosopore remains surrounded by a thick ornamental walll, and thas proved to be a strong point in differentiating different species. It consists of two or three layers, the outermost of which is thick or with many patterns. In A. candila the outermost layer (exopspore) is warty brown. The inner two (mesospore and endospore) are thin layers. The single oospore nucleus is diploid.      

                        https://en.wikipedia.org/wiki/Oospore