You need to first deregister the script using wp_deregister_script
wp_deregister_script( 'admin-widgets' );
then use your code to re-register the script using your own js file:
wp_register_script('admin-widgets', WP_PLUGIN_URL. '/oak-automated-sidebars/oak-widgets.js');
wp_enqueue_script('admin-widgets');
Hope this Helps