Harvey,
My experience matches more with James. A few cultivar mulberry have rooted readily for me, most have not. Some have rooted and then faded, so maybe I just have not got it down yet. Most all callus heavy for me with the exception of Girardi Dwarf, and most places it is listed as Alba, but never seemed to even attempt to root for me. I don’t believe a Alba is a Alba is a Alba. There seems to be inconsistency on species labeling. You mentioned Oscar. USDA Davis lists Oscars as a hybrid, but lists another Oscar as Nigra. At least what is generally available as Oscar is not Nigra in my thoughts. Whitman farms calls Oscar as Alba. Kokuso is a good example USDA Davis calls it Alba, Whitman Farms I believe labels it a cross, Burnt Ridge calls it Morus latifolia. Pakistan according to USDA and Edible landscape is Morus macroura , Whitman Farms and Burnt Ridge label it Alba.
Not that I want to quote Wiikipedia as an authority on fruits, but I do agree with this statement “The taxonomy of Morus is complex and disputed. Over 150 species names have been published, and although differing sources may cite different selections of accepted names, only 10–16 are generally cited as being accepted by the vast majority of botanical authorities. Morus classification is even further complicated by widespread hybridization, where in the hybrids are fertile.”
Then you have Morus Nigra, which I understand is a hybrid that originated somewhere in Asia but is found nowhere in the wild today, and that all our Nigra cultivars are just mutations from each other. It’s different in it’s chromosome count with I believe 308 as opposed to 28 of the other species, and as I understand it’s a sterile plant. The graft failed I did try of my Black Beauty and another scion I received from USDA that was not listed as Nigra, but the buds sure looked like Nigra to me, for what little I know as ID seems complicated.