Lets say I am casting Oblivion Sower. The on-cast effect would activate, exiling the top 4 cards of my opponents deck. The person decides to cast Mana Leak to counter my creature. Assuming I don't have enough mana to prevent the Mana Leak on my own, could I tap the lands, assuming I get three, that are exiled with Oblivion Sower to prevent Mana Leak from countering my creature?
The reason for this is that you will not have those lands when you need to Pay Mana Leak's cost.
Here is the interaction broken down step by step, assuming proper play from your opponent:
- You cast Oblivion Sower, it's ability goes onto the stack on top of it.
- Your Opponent responds to your spell by casting Mana Leak targeting your Oblivion Sower, Mana leak goes onto the stack on top of Oblivion Sowers Ability.
- Assuming no responses, the stack will now begin to resolve the spells/abilities.
- Mana Leak resolves, you must now pay the 3 generic mana or have your oblivion sower countered.
- The on-cast ability of Oblivion Sower resolves, you would now get your lands.
- If you paid the costs for Mana Leak, your Oblivion Sower would now resolve and enter the battlefield.
As you can see, at the time you need to pay Mana Leak's costs Oblivion Sower's ability is still on the stack, so no fresh lands for you (yet). It is also not possible for you opponent to respond to the Oblivion Sower before the cast trigger is on the stack, as no player is given priority when Oblivion Sower is cast until after its on cast ability goes onto the stack.
601.2i: Once the steps described in 601.2a–h are completed, effects that modify the characteristics of the spell as it’s cast are applied, then the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell’s controller had priority before casting it, he or she gets priority.
However your opponent can wait until after the Oblivion Sower's on cast ability resolves and then cast Mana Leak. In that situation you will have the lands, but this would have to be a deliberate choice by your opponent so it is unlikely for that to happen.
An Interesting note to keep in mind is that you will get the lands regardless of whether or not Oblivion Sower is succesfully countered, since the ability is triggered on cast.
It depends on exactly when your opponent casts Mana Leak; he can prevent you from doing so.
Your opponent has 2 options here... cast Mana Leak in response to the "when you cast" triggered ability, or to wait until that ability resolves, and then cast Mana Leak in response to the Oblivion Sower instead.
When you cast Oblivion Sower, the "when you cast" ability will go on the stack on top of the Oblivion Sower. At this point your opponent could cast Mana Leak on top of both of them, and the triggered ability will not have resolved yet, so you will not have those lands.
Note that even if Oblivion Sower is countered, the triggered ability will still resolve normally and you will still get the lands.