King Oyster mushrooms are a species of the Pleurotus family of mushrooms, and related to regular Oyster mushrooms. They have a large stem that confers a relatively 'meaty' or 'hearty' flavor (Umami) and are seen as a medicinal mushroom.
Not too many studies are conducted on King Oyster mushrooms, but they appear to have the same properties of regular bioactive mushrooms in the sense that they interact with the immune system and may 'boost' immunity to sickness. King Oysters seem to be more potent at suppressing fatty acid absorption relative to other mushrooms according to one study, and have been implicated in increasing testosterone 5-fold. This latter study was conducted with very high doses (15-20% of feed) and was conducted in elk (out of all research animals, elk) and has not been replicated.
It also appears to be more estrogenic and bone protective in animal forms of menopause than other medicinal mushrooms tested.