You should look into MINIX 3. This is an operating system that was written in, I believe, less than 10,000 lines. You can get a very good idea of how an OS works with the aid of one of Tanenbaum's books and understanding how MINIX 3 works. You could go straight to Linux, but I think this is a useful task and really helps you see how it really doesn't take that many lines to build a working OS.
http://www.minix3.org/