You can say OK in French.

‘Je suis d’accord’ means ‘I agree’

In spoken French you can say :

1. OK !
2. D’accord !
3. Je suis d’accord [avec toi].
4. OK, je suis d’accord [avec toi].

But you have to write :

1. Je suis d’accord [avec toi].

Source: I am French.

You can say d’accord, but at least half of current usage would be ‘OK’. Throw in ‘okey dokey’ and you’ll lose your audience completely though