I hope that in all those cases, the original characters flowed out of the stories I was writing. I rarely planned those characters, unless the plot would revolve around them, and even when I did, they'd often surprise me. One case in point is my first long Sweetcheeks story, Cherub, where I'd planned for a character to be a "typical" homophobic blue collar guy. And he told me he wasn't. He was uneducated, but he was intelligent and thoughtful and had an eye for art - had he been born to a different family, he'd have been an artist or an engineer or an architect. He doesn't understand his son, who is destined to be a mainstay of SF cons, but that's okay.
I don't write so many OCs anymore. Part of it is that I don't write long stories, but another part is that, for some reason, I haven't felt the need even for my long HP stories. Or, rather, I can borrow Rowlings' - I can make Zacharias Smith a major character and not have to come up with anyone new. I didn't come up with many for Voyager either, and I think it's for a similar reason. They both have large enough casts, with secondary and tertiary members, but they're also in bottles. Most HP stories take place at Hogwarts. New characters are hard to fit in - this is why Rowling has the revolving DADA teacher, after all.
I think if I ever wrote long DS stories (not likely at this point), I'd probably have original characters there, too, just because the plots would probably call for it.
I'm not going to worry about Mary Sues for any of them.