Ignoring the argument as a whole for a second, I believe that Oryx could take the Flood, but only if it was in an early coordinated stage, but that the Flood could infect Oryx due to his technically organic body... Or perhaps the worm inside. I say "early coordinated stage" because the more advanced Graveminds/Keyminds would likely be too advanced for Oryx to subvert, much like Rasputin, only on a far more extreme scale.