You might want to ask ssolie. He writes AmigaOS all the time. Not "for" AmigaOS - the OS itself. The last time we talked, he emphasized that he was using "C" to do it. Since I'm not a programmer at that level (or most any level, for that matter), I'm not sure which "C."
However, I do know that he's currently testing a new SDK for AmigaOS. You might try the AmigaOS wiki
here to find out more. Hope this helps.