Custom reference number on checkout inserts data into wrong order

Question

I've been trying to add a field to checkout where the customer could add a reference number. Right now it sort of works, but my observer grabs the previous order and adds the information to that order in the database. So what i need to figure out is how do i get it to pick the next order / the one being ordered or get some hints on how to do it in a different way. I'm working with Magento 1.9.0.1

Config.xml

    <events>
        <adminhtml_sales_order_create_process_data_before> <!-- Adding reference number in backend working -->
            <observers>
                <customorderfield_backend>
                    <type>singleton</type>
                    <class>customorderfield/observer</class>
                    <method>saveCustomData</method>
                </customorderfield_backend>
            </observers>
        </adminhtml_sales_order_create_process_data_before>

        <controller_action_predispatch_checkout_onepage_saveBilling> <!-- Adding reference number on onepage doesn't work -->
            <observers>
                <customorderfield_onepage>
                    <type>singleton</type>
                    <class>customorderfield/observer</class>
                    <method>saveCustomDataOnepage</method>
                </customorderfield_onepage>
            </observers>
        </controller_action_predispatch_checkout_onepage_saveBilling>
    </events>

Observer.php

    class Lightsolutions_CustomOrderField_Model_Observer 
    {
       public function saveCustomData($event) //Adding reference number in backend working
    {
       $quote = $event->getSession()->getQuote();
       $quote->setData('my_custom_input_field', $event->getRequestModel()->getPost('my_custom_input_field'));

       return $this;
    }

    public function saveCustomDataOnepage($observer) //Adding reference number on onepage doesn't work
    {
        $orders = Mage::getModel('sales/order')->getCollection()
         ->setOrder('created_at','DESC')
         ->setPageSize(1)
         ->setCurPage(1);
        $orderId = $orders->getFirstItem()->getEntityId();
        $order = Mage::getModel('sales/order')->load($orderId);
        $order->setData('my_custom_input_field', $_POST['my_custom_input_field']);
        $order->save();
    }

mysql4-install-1.1php

$installer = $this;
$installer->startSetup();

$installer->addAttribute("order", "my_custom_input_field", array("type"=>"varchar"));
$installer->addAttribute("quote", "my_custom_input_field", array("type"=>"varchar"));
$installer->endSetup();

Answer 1

Thank you @Maikel Koek for your input.

I've modified my code and now it works with our 3rd party checkout. The main difference with our 3rd party checkout system and the standard Magento Onepage is that the standard version submits the data when moving on to the next step. Where as i am able to get the $_POST value from the billing address in the 3rd party extension without any problems.

My code now is as following in case someone stumbles upon this post.

Config.Xml

    <events>
        <adminhtml_sales_order_create_process_data_before>
            <observers>
                <customorderfield_backend>
                    <type>singleton</type>
                    <class>customorderfield/observer</class>
                    <method>saveCustomData</method>
                </customorderfield_backend>
            </observers>
        </adminhtml_sales_order_create_process_data_before>

        <checkout_type_onepage_save_order_after>
            <observers>
                <customorderfield_onepage>
                    <class>customorderfield/observer</class>
                    <method>saveCustomDataOnepage</method>
                </customorderfield_onepage>
            </observers>
        </checkout_type_onepage_save_order_after>
    </events>

Observer.php

class Lightsolutions_CustomOrderField_Model_Observer 
{
    public function saveCustomData($event)
    {
        $quote = $event->getSession()->getQuote();
        $quote->setData('my_custom_input_field', $event->getRequestModel()->getPost('my_custom_input_field'));
        return $this;
    }

    public function saveCustomDataOnepage($event) {
        $orderId = $event->getOrder()->getId();
        $order = Mage::getModel('sales/order')->load(intval($orderId));
        $order->setData('my_custom_input_field', $_POST['my_custom_input_field']);
        $order->save();
    }
}