The Oktoberfest homepage has some of information in English as well. Unfortunately the page about accessibility is only available in German (I'll include the information below).
Going by car is pretty much impossible. Taxis are of course a different matter and perhaps an option to you. But fortunately, the festival ground is very central and easily reachable via public transportation. The three closest stations (which are all equipped with elevators) are:
- The Hackerbrücke station on the S-Bahn - requires some walking and includes stairs, so not good for you
- The Goetheplatz station on the subway lines U3 and U6 - again some walking, but on level ground.
- The Theresienwiese station on the subway lines U4 and U5 - this opens right on the festival grounds (and thus tends to be very crowded) - except the elevator is at the opposite end of the station, so it's about 200m distance.
Finding the way is generally easy - just follow the crowds.
On the festival itself, accessibility should not be a problem, except of course most rides. But the big ferris wheel can be used with a wheelchair. The beer tents are accessible, and there are at least 20 wheelchair-appropriate seats in each tent, but reserved for wheelchair users only until 14:00 on weekdays and till 17:00 on Saturday, Sunday and holidays (for the time afterwards you can get a personal reservation, which is really advisable at all times).
Note that the beer tents are often extremely crowded and hard to get into, especially on weekends and in the evenings. On rainy saturdays, the tents will be full and closed before noon, as people queue up to get in at 10 am and stay until the end at 11 pm. I'm not sure how severe the situation with the wheelchair places is, but unless the "atmosphere" of 10,000 very drunk people dancing and chanting to a mix of folk and pop tunes is the main attraction to you, I'd suggest visiting the tents on weekdays at noon, when they're not crowded and you can have a peaceful lunch.
There are wheelchair-appropriate toilets in the larger beer tents, the subway station Theresienwiese and a service center near that station.
As for the general atmosphere, it's very relaxed and friendly. But due to the copious amounts of alcohol that are consumed, brawls do sometimes occur despite the presence of police and security personnel, and they can get pretty nasty when 1 liter glass steins are everywhere. The statistics for 2012:
- 6.4 million visitors
- 119 cases of aggravated assault (the glass steins)
- 4 cases of rape
- 438 cases of theft