One of the worst things for me in open plan offices is you can hear people eating at their desks. For me nothing is more annoying and distracting than to hear someone crunching on cookies or popcorn or potato chips at his desk. That's an instant headphones-on for me. It's worse than all the other background noise/conversations put together.
I work in China in a semi-open office, every afternoon they make fruit available for consumption. Whenever the fruits are relatively juicy (e.g. Chinese pears), I can't get any work done due to all the smacking and slurping around my work area; I have to go to Starbucks.
I worked in an open office situation once with a co-worker across from me who listened to sports talk radio all day - no headphones. I literally can't imagine a more distracting work environment.
Maybe you could have requested him/her to use headphones. Many a times in a situations similar to above I have requested my coworkers to use earphones, and usually they would comply.
Eventually, and amazingly we actually became decent friends... eventually. In the early days it was a matter of picking my battles, and there were other more important things to work on. (Hard as that might be to believe.) So for a while I just put my headphones in and tried to tune it out.
(Initially I just asked him to turn it down, and I don't recall the exact response, but it was probably approximately, "No.")
What gets me is the smell of delicious food being eaten by my coworkers...I have to stick to plain food in small quantities to keep from getting fat again, and I get jealous...
