WC_Order_Factory::get_order_id PHP Method

    public static function get_order_id($order)
    {
        global $post;
        if (false === $order && is_a($post, 'WP_Post') && 'shop_order' === get_post_type($post)) {
            return $post->ID;
        } elseif (is_numeric($order)) {
            return $order;
        } elseif ($order instanceof WC_Abstract_Order) {
            return $order->get_id();
        } elseif (!empty($order->ID)) {
            return $order->ID;
        } else {
            return false;
        }
    }

 /**
  * Stores information about whether stock was reduced.
  *
  * @param WC_Order|int $order
  * @param bool $set
  */
 public function set_stock_reduced($order, $set)
 {
     $order_id = WC_Order_Factory::get_order_id($order);
     update_post_meta($order_id, '_order_stock_reduced', wc_bool_to_string($set));
 }