The default checkout system inside OroCommerce based on workflow ,So in backend from workflow you have ability to enable  multi steps or one page checkout .

In this tutorial we will show how you can observe the place order which means action finish checkout in wrokflow , So the normal case with symfony is going to services.yml – or any other yaml responsible of DI – and create a service tagged with custom event listener in our case is:

 
- { name: kernel.event_listener, event: extendable_action.finish_checkout, method: onCreateOrder, priority: 10 }
 

the event  extendable_action.finish_checkout is predefined inside orocommerce to observe finish checkout action – and the method responsible  onCreateOrder we should add to our class service the full code service is :

 
    ibnab_eventtest.event_listener.create_order_listener:
        class: Ibnab\Bundle\EventTestBundle\EventListener\CreateOrderEventListener
        tags:
- { name: kernel.event_listener, event: extendable_action.finish_checkout, method: onCreateOrder, priority: 10 }
 

is simple service and the class is Ibnab\Bundle\EventTestBundle\EventListener\CreateOrderEventListener ith content :

 
<?php
namespace Ibnab\Bundle\WhoBoughtBundle\EventListener;
use Oro\Component\Action\Event\ExtendableActionEvent;
use Oro\Bundle\WorkflowBundle\Entity\WorkflowItem;
use Oro\Bundle\WorkflowBundle\Model\WorkflowData;
use Oro\Bundle\OrderBundle\Entity\Order;
class CreateOrderEventListener
{
    public function __construct() {
    }
    /**
     * @param ExtendableActionEvent $event
     */
    public function onCreateOrder(ExtendableActionEvent $event)
    {
        if (!$this->isCorrectOrderContext($event->getContext())) {
            return;
        }
        $orderId = $event->getContext()->getEntity()->getId();
        }       
    }
    /**
     * @param mixed $context
     * @return bool
     */
    protected function isCorrectOrderContext($context)
    {
        return ($context instanceof WorkflowItem
            && $context->getData() instanceof WorkflowData
            && $context->getData()->has('order')
            && $context->getData()->get('order') instanceof Order
        );
    }
}
 

So we have test if is correct order context which has data contain order instance , and inside  onCreateOrder you can write you custom new function for example you can get the order instance $order = $event->getContext()->getEntity() ; 
and make any change on create order .

Other useful event is before create order So OroCommerce give ability to add:

 
- { name: kernel.event_listener, event: extendable_condition.pre_order_create, method: onBeforeOrderCreate, priority: 10 }
 

add the method  onBeforeOrderCreate and make in change to your order line items for example:

 
    /**
     * @param ExtendableConditionEvent $event
     * @throws InventoryLevelNotFoundException
     */
    public function onBeforeOrderCreate(ExtendableConditionEvent $event)
    {
       // your code here
    }
 

don’t forget to add use Oro\Component\Action\Event\ExtendableConditionEvent; to your class,  that is all for this tutorial .