Yep, Doc Oc is definitely going to be in it. British actor Alfred Molina is to play him. I am not sure who the other villains are to be, but I did also read that Bruce Campbell will be making another appearance - hopefully with a bigger part this time.
Osbourn's son does take over his father's role (in the comics), but I don't know whether they would want to pursue that angle right away - perhaps fodder for the third movie though.
There has been speculation over the last week or so that Maguire might not be physically able to take on the Spider Man role for the sequel, and there were reports all over the place that Dunst's real life boyfriend, Jake Gyllenhaal, was to step in. Maguire apparently hurt his back on his latest movie, Seabiscuit. This was all huriedly denied, but there still seems to be soem worry over Maguire's health, so who knows.