If you ever do get it off, I suggest putting a new gasket on before you use it again; it's possible that the old one has rotted in place and glued things together. To prevent this happening again, you should unscrew the base and wash it every time you use it; you'd be amazed (and disgusted!) by how many people NEVER take the base off the jar. After a few months, there is a black gunk forming from old foods that is truly nasty.
I get new gaskets from www.goodmans.net, which has lots of Oster parts (including new jar assemblies if you can't get the old one apart!) for reasonable prices and low shipping charges. But there are lots of other sources; I've seen them for sale in discount stores as well.
Naomi Siegel
May 2006