The H8DCO just has a little bigger center electrode and a little wider heat range than the H8DC. That and the "DCO" plugs are a bit better quality "Made in Germany" part. Bosch moved production of some items to various parts of the world a few years back. Some are fine, some are not. The ground electrode plating on the Made in India spark plugs is not good...lots of bubbles and flakes.
Regards,
Randy D.