Kirchmair et al. (2004) “Phylogeny of the Genus Omphalotus Based on Nuclear Ribosomal DNA-Sequences” Mycologia 96(6):1253-60 provides very compelling evidence that Omphalotus illudens is distinct from Omphalotus olearius based on both sequence data and mating studies. The samples they use for O. olearius all come from southern Europe. O. illudens, on the other hand, was described from the US and samples used in the paper came from the US and northern Europe. Regarding morphological differences " The above paper says: “Kirchmair et al (2002) said that O. illudens can be distinguished from O. olearius by smaller spores and an umbonate pileus”.
Robert Sasata is correct, there are clearly two species here. Besides the Kirchmair et al paper he sites, also see:
Kirchmair, M., Pöder, R., Huber, C.G. & Miller Jr., O.K. (2002). Chemotaxonomical and morphological observations in the genus Omphalotus (Omphalotaceae). Persoonia 17(4): 583-600.
This was discussed in Kirchmair et al. (2004) “Phylogeny of the Genus Omphalotus Based on Nuclear Ribosomal DNA-Sequences” Mycologia 96(6):1253-60. Phylogenetic analysis clearly shows they are different species, despite the outdated opinions of some authors, who could not even agree on the characters that separated the two.
Hmm, I am not sure if the two names represent different taxa (O. olearius is a European name, while O. illudens is an Eastern North American name), or if they are synonyms; in which case I wonder which is given presidence.