test.php

    public function onPreSetData(FormEvent $event)
    {
        $form = $event->getForm();

        $form->add('plainOasPassword', 'text',
            array(
                'required'=>false,
                'label'=>"Oas Password",
                'attr' => array('placeholder'=>'Existing value is set, but obscured for security purposes')
            )
        );
    }