If it was 10 years ago, I could remember what sorts of things TOC was missing but OSCAR had, and give you a bit more useless information; but here's the best I've got.

IIRC, it was mostly anything you needed to get beyond the bare minimum. TOC could message, and manage your buddy list, and set your idle time and your status message; but it couldn't do typing notifications, and probably not some other things. I believe they used TOC tunneled over HTTP(s?) for their Java applet client, if you used that.

I seem to recall it went through periods of unavailability from time to time, so I would go back and forth between TOC stuff I wrote and understood and OSCAR libraries that barely half worked.