Oak???

We have this tree growing just along our garden in the fence between us and our neighbour. It's grown like crazy this summer. In the early spring there was just this nice little tree looking thing, and now it's gone crazy. I'm not sure what it is.. Any guesses? I'm going to guess Post Oak tree?

Close up of the leaves!

Don't pay any attention to the big heart shaped leaves near the bottom. Those are our cucumbers that have attached itself to the tree.