Formal:
* D'accord - This is the most common and formal way to say "okay." It literally translates to "in agreement."
* Bien - This means "good" and can be used to express agreement or understanding.
Informal:
* Ok - This is the most common informal way to say "okay." It is an anglicism, but widely understood.
* Bon - Similar to "bien," this means "good" and can be used informally to express agreement.
* D'accord, pas de problème - This literally translates to "Okay, no problem," and is a more casual way to express agreement.
Other options:
* Oui - This means "yes" and can be used to express agreement. However, it is more general than "okay" and doesn't necessarily imply understanding or agreement.
* Ça va - This means "it's going" and can be used to express that you understand or agree, but in a more casual and conversational way.
The best way to say "okay" in French depends on the situation and your relationship with the person you're speaking to.