Wanted to know which form is better and why
One has to cook himself?
OR
One has to cook themself?
Wanted to know which form is better and why
One has to cook himself?
OR
One has to cook themself?
To be sure, it has to be:
One has to cook oneself
(Oneself goes with one, neither himself nor themselves.)
The above sentence is grammatical, and makes sense. However, it could be ambiguous, as comments have already shown. Overcome that by including a preposition:
One has to cook by oneself
Incidentally, the one - oneself structure is less common these days, especially in AmE.
"One has to cook for oneself" would be good. "One has to cook by oneself" does not mean the same thing; it means you have to cook with no one else around, or no one helping you. "One has to cook for oneself" means that your food (that you will eat) must be cooked by you; no one will do it for you.
"You have to cook for yourself" is a common way of expressing the same idea, at least in informal speech. Depending on the context it could mean either the same as "one has to cook for oneself" (e.g.: 'If you live in this house you have to cook for yourself"), or "you" could actually refer specifically to the person being addressed (e.g. 'Since you are a vegetarian (and the rest of us are not), you will have to cook for yourself').
Note that above I wrote: '"One has to cook for oneself" means that your food (that you will eat) must be cooked by you; no one will do it for you.' Without even being aware of it I switched from using 'one' to using 'you', which feels more natural to me here. I suppose if I wanted to stick with 'one' I could have written: '"One has to cook for oneself" means that one must cook one's own food, no one else will do it.'