I know this is a dead thread, but I've got a bit of relevant info to add. First, I had one that refused to come off with OxyClean alone, just as the OP described. I ended up taking a steel wool pad, dipping it in a saucer filled with rubbing alcohol, and scrubbing it until I got it all off. Took about fifteen or twenty minutes, and lots of elbow grease.
Later I read that vegetable oil will soften adhesive enough to take it off, and then a bit of Dawn or other dish detergent will take off the oil. I haven't tried it yet, but I plan to next time I have a stubborn one.
Finally, I'm hearing GooGone or ammonia might be useful. I will avoid the ammonia; I don't like that stuff and try not to use it if I can help it. But the GooGone is a good idea.