Phantom of Chaos's effect says its Original ATK changes to match that of the card it targets in the GY. This is still its Original ATK.
However, when a card checks for Original ATK (I tried with Great Maju Garzett) it sees Phantom of Chaos's Original ATK as 0, even if the ATK has since been modified (in this case Phantom of Chaos's Original ATK was 5000).

Edit: Tried it with other cards that check Original ATK, each of them saw Phantom of Chaos as having 0 ATK regardless of what it had copied.