Interactions between Oblivion Sower and counter spells that make you pay mana to prevent them?
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.