How can I get a higher mark in my oral exam?

The best way to get a good mark in your oral exam is to practice speaking as much as possible. Try and speak with as authentic a French accent as you can, because there are marks to be gained for correct pronunciation - the best way to develop an accent is repetition until you have gotten a word perfect. You can also learn off some set phrases in case you get stuck in your oral exam - for example, if you are hesitating, an examiner will be very impressed to hear you say "ah, qu'est-ce que je veux dire?", or "excusez-moi, j'ai oublié ce que je veux dire" rather than just pausing or saying "um" until you have found your train of thought again.
  The same goes for learning a few higher-level phrases -  for example, if you are going to talk about sport in your oral exam, you could say that you love football - "j'adore le foot". If you wanted to make this more interesting, you could say that you are a football fanatic - "Je suis fana de foot". Learning off a number of different opinion phrases is also useful, as you can avoid repeating "I think that..." and make your speech sound much more varied and natural - for example, "C'est ma piste de reflexion que", "Je dirais que", and so on.