Since eu- means good and logos word a formal oration of praise is called a?

You're close, but the prefix "eu-" doesn't mean "good" in this case. It actually means "well" or "good" when referring to something being "good" in a specific way, like "well-spoken" or "good-sounding".

The word you're looking for is eulogy, which combines the prefix "eu-" meaning "well" with "-logy" meaning "study of" or "discourse on".

Therefore, a formal oration of praise is called a eulogy.