You forgot and left fully cooked chicken inside the oven it was cooked in for 8 hours before refrigerating it and you would like to know if it is still good to eat?

Unless the oven maintained a product temperature of 140° or more during that time, no food safety person would tell you that it is OK to eat. It could be all right, but there's no way to tell and it just isn't worth the risk.

Addition: but what about putting it in chicken pot pie? see discussion piece for details. thx.