Post Oak

While they may look slightly different all of these Post Oak leaves were taken off the same tree. Even though the leaves themselves might lend themselves well to the name "post," that term actually came from the fact that the wood was frequently used as fence posts due to it being tough and rot-resistant. It's also known as a Iron Oak due to the same reason.
It's a medium tree, growing between 30 and 50 feet tall, and it has the oak look. By "oak look" I mean the grey-brown fissured bark, the twistiness of the branches, the groups of leaves at the branch tips, and the seemingly clumpiness of the leaves when seen from afar. There's the obvious acorns, those give it away too quickly, that are slightly more round than many of the other oak species around these parts.
The cross shaped leaves are what make it an easy identification. They're alternate, simple, 4-6 inches long, slightly hairy on the bottom, and (as you can see) are tough - almost leathery - looking. They can vary a lot even on the same tree, but take a broad overview and it should be simple enough.

Oh, and it should be known that this is one of the more common oaks in Texas. If you can identify this one, Live Oak, and whatever the skinny spiky one is you should be fairly well set.