Magento 1 – How to Run Function When Checkout Page Opens

I am using Magento 1.9.2 and i am working on a custom extension.

My question is:
How can i run custom php function when the checkout page is opened?

I know i have to mess around with the events of the extension config file but i am not exactly sure how i can point the function in the observer which i want to run when the checkout page is opened.

Here is events part of my config:

    <events>
        <sales_order_place_before>
            <observers>
                    <class>opc/observer</class>
                    <method>checkitemsincartv</method>
            </observers>
        </sales_order_place_before>               
        <controller_action_postdispatch_opc_json_saveOrder>
            <observers>
                <hss_save_order_onepage>
                    <class>paypal/observer</class>
                    <method>setResponseAfterSaveOrder</method>
                </hss_save_order_onepage>
            </observers>
        </controller_action_postdispatch_opc_json_saveOrder>
        <sales_order_place_before>
            <observers>
            <custom_ads>
                    <class>opc/observer</class>
                    <method>checkitemsincartv</method>
            </custom_ads>
            </observers>
        </sales_order_place_before>                 
        <sales_order_place_after>
            <observers>
                <newsletter_order_place_after>
                    <class>opc/observer</class>
                    <method>newsletter</method>
                </newsletter_order_place_after>
            </observers>
        </sales_order_place_after>

         <checkout_submit_all_after>
            <observers>
                <opc_checkout_submit_all_after>
                    <class>opc/observer</class>
                    <method>applyComment</method>
                </opc_checkout_submit_all_after>
            </observers>
        </checkout_submit_all_after>

    </events>

And in the observer i have added this:

public function checkitemsincartv($observer)
{
    Mage::getSingleton('core/session')->setItemsInCart("Works");
    $data = $observer->getEvent()->getOrder();
    Mage::log($data->debug(), null, "order.log", true);
}   

But it seems to be not working.

How can i achieve that?