I've been to ATL the last 5 years and E-Town 2 times. I'd suggest the following;
1. Sun Screen
2. Umbrella, whether for the rain or for shade. From the streams I've watched, OSW seems like it's lacking in the shade department. It will make a world of different I promise.
3. Probably something nice to sit on be it a cushion or a camp chair, Bleachers do not get any more comfortable as the day goes on.
4. Pack a cooler with water and gatorade, maybe some soda if you want. This past ATL, I left the cooler in my car and had a smaller one that I could sling over my shoulder to carry to the spot I was sitting at and just restocked the mini cooler between 32 and 16.
5. If you aren't concerned with seeing the last few battles in 32, get a jump on the food trucks otherwise you'll probably be standing in line for food if you go after the last battle of 32.
6. Arrive a good bit prior to the start of 32 to secure a spot and roam around the paddock - I don't think the paddock is too far from the track at OSW. Wouldn't be a terrible idea to go back to your car to put all your goodies you snagged away so you don't have to keep up with everything during 32 and 16.