Caps are an ochraceous-brown colour with prominent darker scales all over.  The stipe also has darker squamules over a pale ochre base; it has a well-developed white woolly ring which usually also has black squamules along the margin and underneath.

It is a serious tree parasite causing the death of many species of conifer, and other species.

other Armillaria species: Bulbous Honey fungus Armillaria gallica and Honey fungus  Armillaria mellea.  (In the past it has been considered to be a variety of A mellea, but is now treated as separate species)

Photograph from top down, in side view and from underneath to show gills and full length of stipe.  Note habitat and substrate.

Parasitic, then saprotrophic on living and dead wood of deciduous and coniferous trees.

Peaking from September to November

Fairly common

common

https://www.first-nature.com/fungi/armillaria-ostoyae.php