How can I make sure I'm achieving the highest grade that I can in French oral?

The key to impressing the examiner in any speaking test is showing a good range; make sure you are demonstrating knowledge of the past, present, and future tense. Even if you only show one example of each, it will increase your grade! Also, try and use some idiomatic expressions to make yourself sound more authentic; 'du coup' instead of 'donc' (meaning 'therefore') or longer phrases, such as 'je suis fidèle à...' (literally meaning 'I am loyal to... but would translate as 'I love' in reference to an activity, or a place).