The "au quai" etymology isn't universally accepted, and isn't recognised by the Academie Française, who don't concede its use.
Generally, people use "d'accord" (sometimes shortened to "d'ac") over "OK" in French.
Generally, people use "d'accord" (sometimes shortened to "d'ac") over "OK" in French.
For all I know, it's entirely possible a misunderstanding/reappropriation of a foreign phrase could be it.