Do some action after WooCommerce order completed

add_action( 'woocommerce_order_status_completed', 'my_function' );
/*
 * Do something after WooCommerce set an order status as completed
 */
function my_function($order_id) {
	
	// order object (optional but handy)
	$order = new WC_Order( $order_id );

	// do some stuff here
	
}