Order Code

observer.php

<?php
            $order = new Mage_Sales_Model_Order();
            $incrementId = Mage::getSingleton('checkout/session')->getLastRealOrderId();

            $order->loadByIncrementId($incrementId);

            /** @var $order Mage_Sales_Model_Order */
            $cookieModel = Mage::getModel('core/cookie');

            // Save order ad origin as order field
            $params = $cookieModel->get('ad_origin');
            if (false != $params) {
                $order->setData('ad_origin', $params);
                $order->save();
            }