Hi Patrick,
They come around with a fuel truck, going up & down the rows of aircraft all week long. This is for 100LL. I don't believe that auto gas is available on the field, but I'm not sure. There are several small airports that make for good stops before you get to OSH. One of my favorites is Portage, WI. They do have self-serve auto fuel there. It's a good one if you're coming from the west. I like to have plenty of fuel on board before approaching OSH, but still top-off after arriving. I've flown my T-18 there from Denver 18 times since the early 90's, and there have been several years where I had to hold due to a problem on the field or traffic volume.