[MUT4NT]

so:

mov ax, [cr0]
or ax, 1
mov [cr0], ax

in real-address mode, it work with immediate values, you are confused.
and yes,it is possible work with 32-bit registers in real-address mode but all depends the situation

[DexHacker]

I would advise you to switch to Fasm and before people jump on my back, there are reasons.
First fasm assembler was made to write a OS, so has many cool things to help you.
Second it very easy to port to your OS (let than half hour, once you have the right function)'
To port nasm you would need to port gcc.
If your going to switch to C code after booting, then stick with nasm.

[Hostwinds]

Bytekill, Check out http://forum.osdev.org/ there are many good OS devs there as well as a wealth of knowledge in their wiki. Goodluck!