You paid for top-of-the-line materials, but he's using the cheap stuff, thinking you won't notice the difference. And the truth is you probably won't. Unless you're a contractor yourself, it's doubtful that you're going to know much about the disparity between oak hardwood floors and cheaper, particle-infused wood.
To make sure you're getting exactly what you paid for, educate yourself. You don't have to become an understudy at your local hardware store, but you might want to learn about the materials you've selected for the project before you start accepting bids from contractors. If you know how oak actually looks and feels, you'll know it's time to send your contractor packing when he shows up with fabricated pulp.