No, I've cum without having an orgasm... did it a few days ago actually... it tends to happen when you're concentrating too much on someone and there's a lot of stimulation at the same time. Plus, it's made worse when you have a female that just "lays there" and expects you to do everyone.

So I say that most of the time "cumming" and "orgasm" are the same thing... but they can be different for a guy.