What to wear? Do I need an Oktoberfest-outfit?
No, you don’t. Really, you just don’t. “Trachten”, traditional costumes have always been part of Oktoberfest, even in its very first edition, the public celebration of the wedding of Prince Ludwig and his Therese in 1810. Also, they have been worn by rural visitors all these years. However, the current costume-mania is a fashion trend, which came up in the 2000s. Munich locals usually refused to wear alpine-style clothing, which has completely changed since then. Today, most Oktoberfest visitors wear some kind of costume, which resemble traditional alpine Bavarian clothing to a certain degree.
You really don’t need to by a pair of cheap Lederhosen or a trashy Dirndl imitation, just to fit in. You certainly should put something on, but the way you dress actually isn’t important. A little advice for men, who are still eager to buy clothes for the occasion: you will often get told to buy checkered shirts, as they’re so traditional. Well, they’re not at all. Get a white one.