-How to avoid ridicule on the basis of the 'arts student' stigma, from other students in more 'practical' degrees
Transfer to science.
That takes my arts students jokes for the day up to about a dozen. But seriously, everyone who says it will be joking, if they're serious then you don't want to know them.
Anyway. Actual answers. Well, if you're doing arts there's going to be minimal maths and chemistry. Since you want to major in Psychology there will be some biology (especially if you do biological psych.) but from what I gather it's not much harder than a high school standard. I'm sure there are some psych majors who'll post here...if there aren't I'll ask my friend for advice, she's doing it.
For Open Day, if you already know what major you're interested in, try to find a student who's doing that. Then just ask them questions, they'll be more than happy to answer. Have a look around Union House, there are some cool places there that will do giveaways or specials (and North Court, next to Union House, will probably have events or live music). South Lawn will have a gigantic set of BBQs and stuff like that, so swing by there for some free food?
O-week...well...I wasn't 18 and it didn't matter. There are enough events happening throughout semester that you'll be able to go out then. The uni pub will serve you without checking ID anyway, so you can go there if you want. And needless to say, the official O-week stuff doesn't require you to be 18.
General advice...work out the locations of classes beforehand so you don't walk around with your diary folded out to the map. Be as social as possible, nobody knows anybody at the start so meet everyone. Then later in the year you can go "oh i remember you" and sit next to them and chat etc. (and it gives you a conversation starter if you like them

heh)