Formal:
* D'accord: This is the most common and formal way to say "OK".
* Bien: This means "good" or "fine" and can also be used as a synonym for "OK".
Informal:
* OK: You can simply use "OK" as it is widely understood.
* Ouais: This is a more informal way of saying "yes" or "OK".
* D'ac: This is a shortened version of "d'accord" and is used in informal settings.
* Bon: This means "good" and can be used as a synonym for "OK" in informal situations.
Other options:
* Très bien: This means "very good" and can also be used as a synonym for "OK" if you want to be extra enthusiastic.
* Pas de problème: This means "no problem" and can be used to express agreement or acceptance.
The best way to choose the right phrase is to consider the context and your relationship with the person you are speaking to.