Oh, wow, no. Those multi-ring combo locks should never be used for anything you care about. My local appliance repair guy showed me how easy those things are to pick when I was in middle school and had one on my favorite bike. And now I'm happy to demonstrate it for anyone I care about who trusts one for anything they care about.
Uh, well, no. You don't "pick" a combination lock. You pick a padlock, like the other ones posted. I know because I've learned how to do it. No lock is secure; I can pick one of those cheesy three-tumbler file cabinet locks in seconds, a medium-grade padlock in perhaps a minute, a good deadbolt door lock takes a few minutes. (Read the book "Surely you're joking, Mr. Feynman" for a description of the basics, and a very good read besides.) There are other, more secure locks like the barrel cylinder locks that Kryptonite uses, but even those can be defeated.
ALL you can achieve with a lock of any sort is to make life more difficult for the thief; how much more difficult depends on just how good the lock is. None of these offer any real security, except for the casual walk-by thief who grabs your helmet because he sees it isn't secured in any way. Which is the great majority of thefts in the real world.
Besides, if someone wants your helmet, all he has to do is bring a sharp knife, like many of us carry around with us in our pockets. The chin strap is far more vulnerable than even the simplest of locks. Slice through it in a second, buy a new one from the manufacturer, basically free helmet. Don't fool yourself that even a very expensive lock will prevent that.