Deck of Wonders and other on-draw effects are not “cast” spells, and as they aren’t “cast” they can’t be countered (as Counterspell reads “when an opponent casts a spell, counter it). The same would affect things here with Tirek, so Tirek would not counter Ambushes, Spider Ambushes, or Scrolls of Wonder.
 
On that note, you’re concerned about this being overpowered? This is so underpowered that it’s weaker than Magma Rager while costing 5 mana more! It dies at a glance if there’s anything on your opponents side of the board, your opponent isn’t going to cast a spell with it out so you’re going to be the only party buffing it (outside of a situation where your opponent has 10 spells in-hand and is really desperate for a minion), and there’s no reasonable way for you to buff it yourself. Let’s be honest, Mages and Warlocks don’t have a bunch of cheap, disposable spells to buff him with and not very many ways to get more cheap spells, and Priest can only do it via Lyra. Even if you are a Priest and you have Lyra out, then it’s only worthwhile if you get 0 and 1 mana spells, as otherwise you’re paying more mana to put a +1/+1 counter on Tirek than the +1/+1 counter is worth. If Lyra ever gives you a high-costed spell like Free from Amber, well congrats! You spent 8 mana to give a single minion +1/+1! That’s terrible bang for your buck.
 
This basically hard-counters Minionless Hunters with weaponless hands and nothing on the board, and that’s about it. Otherwise, it dies before it becomes anywhere remotely close to useful.

@greaterlimits  
Indeed, which is why he’s a 0/2. I had him at 4/4, but quickly realized with cheap spells he’d get out of hand, and without spells, well, it’d be nigh impossible. I might consider even dropping him to a 0/1.

@crazykhmr  
Scroll of Wonders doesn’t seem to trigger things like Yogg, or Quest. Thus I can only imagine it passes counter. Though, counter spell is read in a certain way that it just might. If I ever come across the interaction I’ll let you know. Essentially, Tirek is a counter spell for both sides of the board, and is meant to interact as such.

Not actually that unfair, it also counters your spells too.

question, how would it fare against deck of wonders?