I don't even think an o-ring chain is needed on this bike. It's only around 50HP, maybe a few less, so a standard chain works just fine. Yes, many of us change the gearing slightly, dropping the rear a tooth or two. But that's it, much more and you start losing the "fun factor", the nice "lunge" you get in the lower gears. I like the 33T on my 18" rear wheel. If you have a 16" rear, a 32T will work fine too.