woocommerce-order-status-actions.php

/*

*** Order Status Options ***
>> woocommerce_order_status_pending
>> woocommerce_order_status_failed
>> woocommerce_order_status_on-hold
>> woocommerce_order_status_processing
>> woocommerce_order_status_completed
>> woocommerce_order_status_refunded
>> woocommerce_order_status_cancelled
*/

function site_name_order_complete( $order_id ) {
  error_log( 'Order #' . $order_id ' is complete and has shipped out to the customer.' , 0 );
}

add_action( 'woocommerce_order_status_completed', 'site_name_order_complete' );