I been to that site many times. I use it like a bible. However, if you want to create a GOOD OS you have to create it in NASM first. Then you can port over to C or create your own C compiler. I know its a lot of work, but if you want your OS to be good, you have to do this. That is the one thing that site doesn't show you. I had to figure out all of this on my own. ALSO there are mistakes at that site. For example, it says in one tutorial that the starting memory address is 7C00:000 while another one says it's 0000:7C000 and still another one says 0000:07C0. So wich is it? Well, after some research I found that it's 0000:07C0. But unless you had someone to show you, you would have not known and your code would end up buggy.

Like I said, it's a greate site, but watch out for the errors.