How old was he when you got him? Puppies learn bite inhibition in the litter. If they are taken away too early, you will have to teach this yourself.
You are right that some puppies don't respond to an "ouch". If your pup is very mouthy, it will take some time to train him out of it. First, does he have chew toys? Show him things that he can chew on (toys), and things he can't (you). You can try putting chew toys in the freezer, as this will sooth their gums when they chew.
You have the right idea about nipping - stop play immediately. This is a gentle correction. It may take some persistence, but react as little as possible. Moving pant legs = super fun bite targets, because they are acting like prey. Stop dead.
Most dog trainers now believe that aggressive behavior in adult dogs, like biting, is not a sign of dominance, but of
fear and
insecurity. Again, in a puppy, it doesn't mean much at all. A puppy is not going to be struggling for dominance. Disobedient, scatterbrained, sure, but not trying to be the boss.
Here is an excellent site that uses modern training methods.
Dog Star Daily
Here's a recent blog on the topic of canine manners:
http://www.dogstardaily.com/blogs/whats-it-all