I totally agree with adding more counters to Secret Oath. I came up with some suggested cards that might counter it.
One suggested card: choose a number. Deal damage equal to this number times how many cards in your opponents hand have value equal to this number.
This card could become broken by itself as it comboes very well with 'give your opponent a card'-type spells. However it would be weak without one.
Maybe the mana cost could be equal to the number you're guessing+1.
A second card suggestion:
Your opponent must discard the highest card in his hand. Deal damage equal to its power. Your opponent must randomly discard a number of cards of your choice up to the damage dealt with this card and draw an equal number of cards from the deck. Card costs 2 mana cards to activate.
This is very similar to the 'steal the highest card in your opponents hand and deal damage equal to its power'-card with the difference that you don't gain a card but instead can make your opponent re-draw cards.
Edit: Or maybe just deal 1 additional damage instead of re-drawing a number of cards.
A third card suggestion: your opponent draws 3 cards and then gives you the highest mana in his hand. Deal damage equal to how many more cards than you he has. Card costs 2 mana cards to activate.
Fourth suggestion: 4's are worth 5 on your turn and 3 on your opponent's turn. Ongoing effect.