I did French last year, and you're right about the oral - you can really prepare for it!
Obviously it probably varies with the actual person you get - but from what I've heard the majority of them are really nice and they follow a logical pattern with their questions, which is probably what makes it possible to anticipate what they'll ask next, and prepare to be asked it! For example, when talking about how I planned to go to Monash after finishing Year 12 they asked me things like, why do you want to go that university/what are you thinking about studying and why.
With the written exam, it's true that it can be on anything. That said there's probably broad topics that usually come up more than others: holidays, school, French culture, that kind of thing.
Our exam last year had a listening text about going to a carnival, and another one was something to do with French cuisine - it really varies! But yeah as alondouek said, one thing they won't do is chuck in a text with a lot of technical words/jargon that will leave everyone completely confused. A lot of it is usually using quite general language, but of course, the more vocabulary you know, the better! Also you have your dictionary to look up any key words you don't understand. (Use the time wisely though!)
In the writing section especially there's usually a range of topics and text types - I think in most years there's a 'personal' topic ('letter to a friend' kind of thing) which you should be able to do without having to look up/know any words specific to a certain topic (e.g. the environment).
If you happen to know all about a particular topic that one of the texts is based around, then that's great of course! But in the end, I think the key is to expose yourself to as much French as possible and ideally you'll be able to adapt to give the exam, no matter what topics turn up, your best shot

Hope this helps a little!