I would like to comment that it, in fact, doesn't. I've run Opera on OS X and Windows for a few years now and have seen no indication of that. In fact, I can see only one Opera process in Activity Monitor right now, with 15 threads - even if I open up a Youtube video. When Flash crashes, so does the whole browser (which used to happen all the time with the 10.5x betas). I've heard rumors on the My Opera forums that Opera on *nix might have this, but the OS X version certainly doesn't and I have no knowledge that the Windows version would either.
Opera is a great browser, but this is something I've yet to see (and am eager to).