Not a ton different, if you've used OPatch already. Basically it copies around files, runs a bunch of perl, runs a bunch of sql, and then you run the usual patch upgrade scripts at the end. The in-place patching is safer and smoother in 12c than it was in 11g, but the cumulative patching has really gotten out of hand.

For example - on a QA system we were running a couple of custom one-off patches delivered by Oracle Support. These blew up the entire patch process. I had to stop, back them out, re-patch, then re-apply the one-off patches.

Oh, and 12c is a seriously whiny bitch about XDB. Just for added fun.