So what the heck does うおォン mean? I can find citations of it on the internet, many but not all in context of K-on!, but I can't find anywhere reliable that gives me a definition. The mix of hiragana and katakana is odd, but I would guess meaningful somehow.

It's a reference to the manga 「孤独のグルメ」 (Kodoku no Gourmet). In Chapter 8, the main character is eating 焼肉 (yakiniku) and suddenly exclaims, 「うおォン、俺はまるで人間火力発電所だ」 (Uoon, I'm practically a human [power-generation] furnace!) Due to the oddness of the sound, it apparently became a meme.

First and second google results for "うおォン" are both relevant.

Interesting. Thanks for the clarification and translation fix.

I'll have to add niconicopedia to my list of references.

By the way, ゲン担ぎ (弦担ぎ) refers to a superstitious action performed as a good luck charm. Often used to refer to food, I think - like eating a particular dish before attempting to take an exam, or run a race, etc. I think she's literally trying to eat the string because Ritsu said that it worked for her in the past, or something.

Oh, and looks like there was a pun we missed too. Heh.

Ladies and gentlemen... I believe we have stumbled across the most untranslatable joke in history.

Nah, post #233758 was worse.