I would say, and this is obviously my opinion, that for any card(s) to be considered OP it/they would have to meet a certain criteria. That criteria being "can it/they be played around?". If the answer is yes, then the card(s) can't be OP because you can easily minimize the effect they produce on the game. If a card or combination of cards can't be played around and their effect(s) minimized then I would say they are OP. To Blizzards credit, I personally haven't seen anything that I would consider OP.
As an example, Unleash the Hounds, which was recently nerfed because it was widely considered OP. But was it? If you maintained a board of no more than 1 or 2 minions, then at BEST a hunter could have produced on turn 10, Buzzard (2 mana), 4 hounds (2 UtH, 4 mana), 1 Timber Wolf (1 mana), and summoned an Animal companion HOPING for Huffer. All said and done they would have played 5 cards, drew 6 cards, and IF they got Huffer they could have done 13 damage. That is if your board was 2 minions. So, yes I would say it was OP at 2 mana because even if you attempted to play around it they could produce 13 damage, and draw 6 cards in 1 turn. So you attempted to play around a combo of cards and it was still SUPER powerful.
Pyro + Equality, if you do the same thing and maintain a board of no more than 1 or 2 minions is not OP because at best it is an even trade of 2 cards for 2 cards. At worst, it would be 2 for 1. You just have to learn to play around combos that SEEM super powerful, because if you do, you will notice that those combos don't seem as good any more.