I put a copy of your comment about distinguishing between A. mellea and A. ostoyae on the name pages for each species along with some of the bits of my comment.

In California, both Armillaria mellea and A. ostoyae have a thick, felty annulus and they are the only Armillaria species here to have that character. They differ by the color of both the cap and the annulus.

A. mellea has a white to yellow edge on the annulus, the cap is honey yellow and the disc is darker than the margin giving it a two-tone appearance. The cap is smooth and the hairs are indistinct.

A. ostoyae has a brown edge on the annulus, the cap is brown with dark hairs.

What rule do you use? I generally use A. mellea for the ones that grow on hardwoods and A. ostoyae for the ones on conifers. I do know that Tom Volk id’ed a collection from southern California growing on hardwood as a slightly unusual form of A. mellea. I also sent him a sample from Santa Cruz that came out as A. mellea. Apparently the California material mated with the true A. mellea, but it was not a typical mating. Macroscopically I remember Tom noting that the California material has a scalier cap than classic A. mellea. Based on his key it looks like we should be doing more microscopy and looking for clamps at the base of the basidia. Personally I have a hard enough time find a good basidia, much less figuring out whether there’s a clamp at the base. Have you had much success with that? Any recommendations?