Trigger event after an order has been created/saved

you can use the event checkout_onepage_controller_success_action at Namespace\module\etc\frontend\events.xml :

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
<event name="checkout_onepage_controller_success_action">
    <observer name="mymodule_controller_success_action" instance="Namespace\Module\Observer\MyObserver"  />
</event>

Observe the sales_order_place_after event instead.