Labrador puppies definitely go through bratty stages. If Oliver's gotcha day was 9/11, does that make him about 5 months now? That's a common acting up time. Mostly, they are just checking to see what they can get away with. You're doing the right thing by not giving in. He might also need more exercise (another sign of a healthy growing lab).
Just an fyi- labs tend to go through "teenager"dom at about 6-7 months, 9 months, and 12 months, too. If you stay firm and keep with your established routine, these phases usually don't last terribly long. But it's a good idea to learn about Nothing in Life is Free and start using it: Dog Training: Nothing in Life is Free : The Humane Society of the United States
One of my stand-by practices when Kimber was bratty was to feed her meals to her by hand, making her do a short command (usually "leave it") before every piece of kibble. It only took a couple of meals for her to settle down again.