How to get Last OrderId inside an Observer

Get the latest order_id from the session like this:

Mage::getSingleton('checkout/session')->getLastOrderId();

Here is also another way to do it.

To get the last order ID in Magento:

   $orders = Mage::getModel('sales/order')->getCollection()
       ->setOrder('increment_id','DESC')
       ->setPageSize(1)
       ->getFirstItem();
    echo $orders->getEntityId()