Although the focus for many is on the beer at the Oktoberfest, if you want to survive you must eat almost as much as you drink. Just like at any major festival, there are lots of options to choose from.
In the Tents: All of the tents have traditional Bavarian food for breakfast, lunch, and dinner. The prices are high, but the food at all of the major tents get impressively good reviews year after year. Some of the tents have specialties which are unique at the Oktoberfest. For example, the Ochsenbraterei specializes in dishes with Oxen.
Here are my recommendations for inside the tents:
For Breakfast: The traditional thing to eat is Weisswurst (white sausage) with sweet mustard. You should be able to find this on the menu at every single tent.
For Lunch: Pork or chicken. Forget about sausage; notice how the Germans are not ordering sausage?
For snacking in the tents: Radish or Pretzels and Obatzda (a fresh cheese spread)
For Dinner: Pork or chicken. Forget about sausage; notice how the Germans are not ordering the sausage?
Street food: There is food everywhere at the therisenwiese (Oktoberfest grounds), much of what you will find outside of the tents is less traditional and more along the lines of typical food at carnivals anywhere in the United States, for example. However, you can find dumplings, fish on a stick, rotisserie chicken, and roasted nuts among hundreds of other options on the streets between the tents.
Actually, my personal favorite can only be found in a tent, but is self-service only so many eat it outside the tent. This is an Oxen sandwich and can be bought only at a counter just inside the door to the Ochsenbraterei on the North side of the tent. If you approach the door from the outside, you will see people eating these in the entryway as they are quite popular amongst Munich natives.