Oosphere is a female sex cell that makes up the plant egg. To originate the plant embryo, there is a fusion of the oosphere with one of the pollen grain generator nuclei during fertilization.
The female gametes are determined within the plant egg to fuse with the male gametes and give rise to the zygote. In spermatophytic plants (angiosperms and gymnosperms) - those that produce seeds - the zygote resulting from the fertilization of the oosphere is transformed into an embryo that is found inside the seed.
The zygote produces the sporophyte, the diploid phase that produces spores. The female spore is called megaspore (mother cell of the egg) and the male spore is called microspore (corresponding to the pollen grain).
When the pollen grain disperses (pollination) it encounters the archegon, which is a female reproductive structure where the female gamete (oosphere) forms. In this process, a pollen tube forms and releases the anterozoids that will swim to the archegon and fertilize the oosphere.