Magento call observer after order save commit as below.
Syntax of Magento call observer after order save commit
Add the following lines of code in your config.xml with details of your module.
<events>
<sales_order_save_commit_after>
<observers>
<namespace_modulename_model_observer>
<type>singleton</type>
<class>Namespace_ModuleName_Model_Observer</class>
<method>Name_of_method</method>
</namespace_modulename_model_observer>
</observers>
</sales_order_save_commit_after>
</events>