Formal:
* D'accord - This is the most common and formal way to say "okay."
* Bien - This means "good" or "well" and can also be used as a synonym for "okay."
Informal:
* OK - This is an English loanword that is commonly used in informal settings.
* Bon - This means "good" and can also be used as a synonym for "okay."
* D'accord, d'accord - This is a more emphatic way to say "okay" and is often used in casual conversation.
Other variations:
* C'est bon - This literally means "it's good" and can be used to mean "okay" in certain contexts.
* Oui - This means "yes" and can sometimes be used to express agreement, similar to "okay."
The best way to say "okay" in French will depend on the situation and your level of familiarity with the person you are speaking to.