Formal:
* D'accord. (This is the most common and neutral way to say "OK, I agree.")
* Je suis d'accord. (This is a slightly more formal way to say "I agree.")
* Je suis entièrement d'accord. (This means "I completely agree.")
Informal:
* OK. (This is the most informal way to say "OK")
* D'accord avec ça. (This means "OK with that.")
* Pas de problème. (This literally means "no problem," but it can be used to mean "OK.")
* C'est bon. (This means "that's good" and can be used to express agreement.)
In a specific context, you might use:
* Je suis de votre avis. (This means "I agree with you" and is used in a formal context.)
* Je partage votre point de vue. (This means "I share your point of view" and is used in a formal context.)
The best way to choose the right phrase depends on the context and your relationship with the person you are speaking to.