It seems to me, that if fish is as abundant in your metagame as you describe, why would you not run Oath as your Deck X and then sideboard into a different strategy.
QFT. Start with oath and switch to something else.
If you're trying to beat fish specifically, I think that switching to Gro or Dragon would not be the best way to go. Gro is still a creature based strategy, so they will probably keep their STPs and possibly bring in even more creature hate. Dragon, although a different strategy, is very much hurt by both creature removal and enchantment removal, so their sideboard cards will still work very well.
I would suggest playing an oath list similar to GWSOath (linl)(not the most current list, but a decent starting point), then switching to something like storm combo where you SB something like:
-4 Oath
-4 Waste
-2 Angels
-1 Blessing
-?? Leak
+4 Dark Rit
+?? Cabal Rit
+1 YagWin
+1 Necro
+1 Tendrils
+?? other stuff
This way, they would sideboard creature/enchantment hate, be naming the wrong thing with meddling mage, and have a grip of dead cards against you. With fish's slow clock you should be able to sculpt a hand to win through some Force/Stifle with your own forces and duress.
I think there might be an article/thread about this somewhere on starcity or here on TMD.
Anyway, hope that helps. Goodluck. Transformational sideboards kick ass.
-Joe
