Does the effect of Buster Blader power up for both dragons on opponents field and graveyard or only one of is targeted because it says gains 500 for on field OR graveyard?

Answer 1

It is oddly worded if you think about it, but it does gain ATK for dragons in both the opponent's graveyard and their side of the field. I think the 'each' part is more important, the phrase could be worded as "For dragons that are on the opponent's field, or in the opponent's graveyard, this card gains 500ATK for each".

Answer 2

A dragon monster cannot occupy both graveyard and field at the same time. That is why the 'or' is being used. It is not being used in an exclusionary sense. Think of them like logical operatives - AND returns 'true' if the object meets both conditions, while OR returns 'true' if the object meets either condition.

Think of it like this. Buster Blader's effect will consider each opponent's dragon monster with the following statements;

"Are you in the opponent's graveyard?"

"Are you on the opponent's side of the field?"

If the answer to either is 'yes', then Buster Blader gains 500 ATK from it.